1) Bugs are likely to be fixed more quickly
2) We're likely to get access to things like DAV, SOAP, and whatever the next cool technology is going to be.
3) It might make it easier to run OpenACS alongside some other technology, something that I believe people will want.
4) Running under Windows
5) Easier installation, because people are likely to have Apache running already.
That's what I'd be interested in, anyway.