Solid Hardwood, Engineered and Laminate Flooring - Hardwood Flooring Underlayment Questions

I'm going to install 3/4" solid oak 2-1/4" strip pre-finished flooring, and have questions about underlayment. We live in the desert so it is very dry here. The sub-floor is plywood, and the room is second story over a garage so noise below is not a problem. The flooring will be nailed.

The flooring store told me that underlayment is unnecessary in my situation, and they suggested it should be nailed directly to the plywood sub-floor. If there is no good reason to use it, I'd prefer to skip the step. I don't see any problem with moisture coming from below. The only chance of serious moisture from above might be flooding from plumbing. Would the underlayment really protect the sub-floor in this situation? What other purpose might the underlayment serve?

If you still think I need underlayment, what should I use, and where's the best place to get it?

Also, just curious - does pre-finished require less time to acclimate, and is 72 hours OK?

Thanks in advance.

I'm not a flooring pro, but I would be real worried about floor squeaks if you lay the floor on 1/2" plywood. W/O an underlayment I think you'll get a springy floor. Underlayments provide a smooth surface, stiffen the floor and bridge subfloor seams.