If:
- it's an easily replicated piece - like a facing or cuff (I actually find collars hard to draft so I would avoid it if it was missing that)
- a piece I didn't need (like I wanted to make a jacket and the skirt was missing from a suit)
- I wasn't "collecting" that kind of pattern (for instance, I collect Hollywood patterns, not just to sew, but to have. So in that case, I want one in as nice condition as possible)
- it was very reasonably priced (for instance, in the case of a suit missing its skirt, I would kind of want to pay *less* than half the price the complete suit would have commanded)
Then, I'd consider buying a pattern with a missing piece.
But you never know, sometimes there's a specific pattern or a design that I desperately need, then I might be happy to find it even with a missing piece.