The guys who only use nails say that hardwood floors need to be able to shift during winter and that using glue would cause them to crack.
Different humidity in different seasons can cause floors to contract and expand. This logic is fine. The amount of contraction/expansion will depend on your climate. Some floors should be installed "floating" because of this, i.e. not connected to the subfloor. Some floors should be installed rigidly connected to the subfloor, if the subfloor will expand/contract too, or if the connection to the subfloor can resist the movement and just cause acceptable tension/compression in the floor.
Coming from a logical viewpoint (and with no experience of hardwood flooring) I don't see how there is any difference in gluing vs nailing in terms of allowing contraction/expansion. Either the hardwood floor is connected to the subfloor or it isn't. "Nails are good by glue will cause cracks" makes no sense to me.
The best thing to do is follow the manufacturer's instructions. Or if you're not supplying the hardwood floor, allow the installer to do it however he wants and sue him if it doesn't work.