If you had no holes in the shell, the note you create on the drums should sound and then lower in pitch over a second or two. That's due to the pressure within the shell and how it engages the heads fully. No air would escape the shell (or, very little would leak out, perhaps), so it will sound quieter in general, because the only sound waves coming off the kit are coming straight off the heads. With a thinner-shelled tom, the shell itself would resonate as well.
The more holes you add, the more air escapes after the initial attack of the stick, and there's less resonance. It may sound louder because there's more air escaping (in the directions of the holes), but you sacrifice that energy being passed back and forth between the heads and through the shell--there goes your tone!
If you check out some of Spaun's shells they have out now, they can have a few huge holes or many small holes. They'll be crisper and drier sounding. They also will be a bit louder, but that loudness won't sustain over time like it will with a less-vented snare.