I rented a Specialized Safire from my LBS before I bought my old Gary Fisher. Totally ruled everything about the bike out as far as fit. None of this was very obvious in the parking lot. I test rode my bike in the parking lot before ordering and felt like it was an okay fit but wasn't sure on the 29er, so I rented one (well the HT version). By the time I returned it I was sold and realized it was the best fit I had ever rode (did almost 40 miles of trail in 24 hours).

On my road bike I did a ride on a hilly road near the shop (a different local shop that didn't rent) and thought it felt right. Then on a long ride within the first month or so I was in terrible pain. After multiple bike fits it turns out the top tube is actually a bit too long and the bike will probably never feel dialed in. It makes me so sad and so frustrated to have a bike that I was excited to get and find out I was wrong because they wouldn't allow a long test ride.

Not sure what your reservations are exactly about renting but I think it is a wonderful idea and will do it if possible on ALL future bike purchases.