Well, if the tom isn't hard panned, folks on the left won't miss it. Plus, (and this is very important,) the sound person should always go out into the audience to hear if there are dead spots. There should be nowhere in the entire venue where something in the right speaker would not be heard in the left. Now, I realize, in some venues, this is simply not possible. The deader the spots, the softer the pan. If there is a marked difference in what can be heard where, all panning may have to cease.
But if it is a box-shaped venue, and there is good speaker placement, panning is not only possible, but it provides vastly superior sound, as there is movement. Just as we don't want two things to occupy the exact same frequency, we do not want them to occupy the same physical space. Just as you listen to headphones, its good to put certain instruments wide out, (both left AND right,) lead vocals down the center, and harmonies slightly panned. Kick drum and bass can still inhabit that center channel, but since guitar uses a lot of the same frequencies as vocals, it's nice if it is slightly different in how its placed.
I mean, you can soft-pan by only a few degrees and radically alter a mix. To me, it makes no sense at all to mix in mono, unless the room limits you to it.
The thing about mixing in stereo is that its easy to muck it up. But if its done correctly, it provides far more interesting sound, and will make the audience a lot happier, and they won't even know why.