We're glad that the installer that suggested that you not repair the loose subfloor isn't in the bridge building business, otherwise we'd likely being looking for alternatives for travel. There are two specific issues that need to be considered here; the first is the irritating noise that is created by the loose boards and the other is the relative integrity of the floor itself.

If the major problem is the squeaking floor than this is a simple repair that can be achieved by driving hot-dipped galvanized finish nails right through the carpet, pad, and subfloor and into the floor framing members (floor joist) in the vicinity of the noisy area. You'll want to be sure to use a nail punch to counter-sink the nail head slightly below the surface of the subfloor to avoid future injury.

If on the other hand you are still experiencing some give in the subfloor after installing the nails then chances are that you need to provide additional backing to nail the subfloor to. This can be especially easy if the floor framing is accessible from below and will not require the removal of the carpet. If the floor framing is not accessible then the most efficient way to solve the problem may be to pull back the carpet and pad in the area affected in order to repair of replace the damaged subfloor. You can then reinstall the carpet.

Contact a handyman or small repair contractor if this sounds like a project that you aren't prepared to tackle.