If you're talking about using construction grade ply then it really depends on how far it's short comings could be compensated for in the other components. Things are different for guitars compared to say, basses. Less string tension so potentially guitars could use cheaper materials with less compromise on sound given for most guitars a lot of the character comes from the pickups.
I'd say slap a set of EMG's on a ply body with a carefully laminated neck of say soft mahogany with some maple for structural stiffening and a richlite fingerboard and it might sound OK.