The sonic difference betwen most run-of-the-mill pickguards is arguably indistinguishible. "Arguably" because perhaps one person in a room of 50 could maybe tell the diff in a blind test, but then again, who else would care
The use or not of a pg for most folks really is either functional (because your style will mar the finish), or aesthetic.
Then again, the Gibby hummingbird, let alone the double pickguard on some of their models, are humongous! With that much plastic solidly connected to so much real estate of the soundboard I'd not be surprised to hear that there are folks who have a sonic preference. That said,
having a pickguard on such a guitar may just well be
part of that guitar's sonic signature.
In all, to each his own. But to suggest that all pickguards detrmentally affect a guitar's voice is, IMHO, a gross gereralization.
Edward