The only thing I can think of its that there are a few schools that have both and the MFA program is better regarded. For example, Pace and the Actors Studio (the MFA program). But even in these, the competition is more a question of where school resources go and not a direct competition between students. The situations are also not very common. My daugther is applying to 14 schools and not one of them has an MFA program that competes with a BFA.
In performance it can be an issue if the performance opportunities are shared between undergraduate and graduate students. It can be an issue in design and technology if the design opportunities are shared between undergraduate and graduate students. At schools like Pace and Yale the graduate and undergraduate programs are completely separate, so those would not impact one another.
It is important enough that you should ask about it, but just because it exists doesn't mean it is automatically a bad thing. As Kat says, the best situations we found were where undergrad and grad programs were very separate. The same goes for schools with both BA and BFA degrees.