My school just recently formed a premed committee and I missed the deadline to get the committee letter written for me. So, I panicked and asked around if I was SOL or not. Turns out not. I individually solicited professors and supervisors to write them, provided them addresses, and the profs sent them out to the 11 schools that I ended up submitting secondaries to. A couple of them expected me to provide them with pre-addressed and stamped envelopes.
Now that the secondary part is all said and done, I'd have to say that I wasn't hurt by not having a committee letter. I interviewed at just over half the schools I sent secondaries and have been accepted to at least two programs.
There is a rumor that if there's a committee and you don't use it, you may be asked why not. I've never actually been asked this question, but I suppose that it could have played a role in my not getting more invites. I dunno.