Even then, I usually comment out partial code then slap on a TODO. And where it makes sense, I'll also add in a "throw NotImplementedException" or some equivalent, so it's explicit that the code isn't expected behavior when executed. It's usually not much effort to do this. Especially when you plan ahead while you write.
There's always repercussions to checking in unbuildable code. Suddenly, nobody else can contribute or pull from that branch without first fixing your bad code, for one. What's worse is when they think it's a mistake and do tweak your code. You'll have to backtrack then, adding noise to the file's history. Another is when you need to roll back. If you allow broken commits, there's always a change of hitting a broken version. In times of emergency, you really don’t want to be bogged down by unbuildable code.
There's always repercussions to checking in unbuildable code. Suddenly, nobody else can contribute or pull from that branch without first fixing your bad code, for one. What's worse is when they think it's a mistake and do tweak your code. You'll have to backtrack then, adding noise to the file's history. Another is when you need to roll back. If you allow broken commits, there's always a change of hitting a broken version. In times of emergency, you really don’t want to be bogged down by unbuildable code.