Amy, I really do appreciate your perspective as an AD.   I know that budgets are often very tight, and most places can't afford to pay more than one entertainer a week.

I do run into ADs who choose to use only volunteers, and in truth, SOME volunteers are top-notch musicians. (E.g., I know some excellent bluegrass groups and players who play for nursing homes as volunteers. Then there's the wonderful Boeing Choir in the Seattle area, who perform as amateurs despite being very high "professional quality.")   But there are two problems with relying on volunteers:
1) Sometimes people who volunteer don't see the booking as a commitment in the same, serious way as they would a paid gig.    I'm told volunteers are "no-shows" far more often than paid performers are.
2) Few entertainers can afford to do a LOT of unpaid gigs.   Hence, it's hard to book all the desired spots with volunteer entertainers, especially high-quality ones.   
(If you do a gig for "business," you can write off over 40 cents per mile for travel, for example. If it's as for "charity," your mileage deduction is only about 15 cents per, and if it's as a "hobby, you can't write off any expenses at all.)