But couldn't the benefits package be an important factor in someone's decision whether or not to wait for a potential offer from employer XYZ ?
If I'm well qualified and have 2 offers in hand and am waiting on XYZ to hand me a potential 3rd offer, time is running out on the two offers in hand, and I want to know - how many sick days am I going to get, are they going to pay for my MBA, what kind of insurance plans, etc ... before I turn down the offers in hand.
I think it's a totally fair question to ask before an offer is even made. And, I also think it's ok to ask the interviewer (who may not be HR), because the interviewer is the temporary liaison between the candidate and the company. The interviewer is the representative, so if he doesn't know what the benefits package is, he needs to find out from HR and let the candidate know (or at least point in the right direction).