First of all, I want to stress that Wizards wants people to play with foils. They like seeing foils. They do not want judges to discourage foils. In fact, the Wizards folks make that point fairly emphatically.
You can play with foils.
That said, there are some foil related issues that need to be discussed. In short, these issues all relate to having – intentionally or not – a marked deck.
Having a marked deck means that some cards in the deck are easily distinguishable. That is a bad thing. As in game-loss or match-loss or throw-you-out-of-the-tournament-and-let-the-DCI-investigate-you bad thing.
Foils are easily distinguishable. They bend differently, they curl, and they are heavier than normal cards. These problems are more pronounced in older foils, like those from Saga and Masques blocks.
First, foils warp, especially in humid weather. If you don’t believe me, just put a couple foils on a table some humid day and come back a couple hours later. The foils will have a significant curl. If you put a couple normal cards next to them you will notice that the foils warp far more than the ""normal" cards.
Second, foils bend differently. The metallic skin means that one side of the card is less flexible. This difference is very slight, but if you do nothing but face down rifle shuffles, for example, and especially if you bend the cards a lot during riffles, this will result in your foils having a different curve than the other cards.
Third, foils have a different weight from normal cards. Once again, this is more of a problem with the older foils. When Urza’s Legacy (the first set with foils) hit the streets, urban legend has it people were weighing the packs with cocaine scales to identify which packs had foils.
If you are playing a couple significant foils in an otherwise unfoiled deck, this can result in a marked deck. The extent, and the impact of that problem, depends on just what the marked cards are and how significant the markings are.
Simple advice: if you want to play with foils, do so. Just make sure you have a mix of foils and non-foils, or all foils. For example, I did a deck check on a guy playing Tooth, with the following foils; one Tooth, a couple Elders, some Forests, a couple random Urzatron components, 2 Sylvan Scryings, 3 Reap and Sows and a Triskelion. The rest was non-foil. My response when I gave the deck back: "Nice deck. No problems. You have an extra ten minutes for the match." (The extra time was because of the time taken up by the deck check.)
Here’s another example, also from the Minneapolis Regionals. A player had an entirely foil Tooth deck, including some foreign foil rares – except that the four Sakura-Tribe Elders and the three Eternal Witnesses were not foil. This raised all kinds of red flags. Having some particular non-foils could be evidence of cheating. For example, if he was feeling his way up the deck, then cutting, that could be because he was trying to cut to an Elder or Witness. That would have been grounds for immediate disqualification. What really stood out, however, was that the non-foils were cheap cards. Who can get a playset of foil Tooth and Nails, but can’t get a single foil Tribe Elder?
However, I had watched the player earlier, from off to one side and behind him. His shuffles and cuts were clumsy, and his play a little weak. What I had seen of his previous actions, when he was almost certainly unaware I was watching, made me question whether he would be cheating. I talked to a local judge. He knew the player, and knew that the player had borrowed the foil deck from a friend. The friend had an entirely foil Tooth deck (some kids have way too much money!), but audibled into a R/G deck at the last minute. The friend kept the foil Sakura Tribe Elders and Witnesses, and loaned the rest of the foil deck out. That was pretty good evidence that the non-foils were there for a legitimate reason, not to allow cheating, and my discussions with the players supported that idea.
Between rounds, I talked to both players. I explained that, based on the contents of the deck, we judges had seriously discussed disqualification without prizes, but we were not going to penalize them in this case (okay, technically, the discussion was a caution, but. whatever.) I hope we educated them about the problem. (On the flip side, if the player really was cheating, it should indicate that he had been suspected, albeit not caught.)
Another example from Minneapolis Regionals: a player had just one foil in his mono-Green Tooth deck – and the foil was Kiki-Jiki. This one also caused some discussion among the judges. Kiki-Jiki is the one card you never want to draw in a Tooth deck – you generally want to search it out with Tooth. Because foils bend differently, we were concerned that the player was using the foil to increase the chance to cut to Kiki – thereby putting it on the bottom of his deck after the cut. Several judges tried shuffling and cutting the deck, but we did not cut to Kiki. However, for the rest of the tourney, some judge was watching how that player cut his deck whenever possible. We never saw anything suspicious, but I did talk to him afterwards and tell him what was going on. My advice was simple – I didn’t think he was cheating, but he should change the foil mix, just to avoid suspicion.
A final example, from the JSS Championship: a player running mono-Red had just two foils in the deck – both Urza’s Saga foil Sowing Salts. Those were the only Sowing Salts in the maindeck. Now JSS Champs was a Standard event, and Urzatron Tooth was a Tier One deck. Sowing Salts is critical in the mono-Red / Tooth matchup. The old Urza’s block foils are both heavier and bend more than newer foils. In the event, when we did the deck check, the foils were slightly warped. I could shuffle the deck and cut to a Sowing Salt maybe half the time – and I’m clumsy. Other judges could, too. He received a match loss for marked deck major, and had to replace the Sowing Salts with normal cards before he could continue. (Note – after discussions with the player, the conclusion was that this was not deliberate cheating, so he was not disqualified. However, even accidentally, the deck was still marked in a significant way, hence the match loss.)
Playing foils is fine. Just take a minute to think about the mix of foils and non-foils in your deck. Is it is a random mix? If so, great. Can you summarize the make-up of the foils in a single word or phrase – for instant the foils are "all the lands" or "LD" or "the win conditions"? Those are problems. If you have that problem, add some more foils, or subtract some. Just make sure the mix is obviously a random mix, and you will have no problem.