I'm not sure if this is still the case, but it used to be that if you had any foils in your deck and took it to a tournament, you had to either foil the entire deck, or replace all foils with non-foils. This became problematic for cards that were only available as foil, like M19 Nexus of Fate (from what I remember).

Personally, I only play casually anymore. Foils don't matter to me at all. If a card is foil, then it's foil. If it isn't, then it isn't. I only care about its playability.

Some foils do bend really badly, but I've been fortunate enough not to run into that problem yet.