I'd agree with you if we were talking about an actual computer game, but... this isn't a computer game. It's just a gaming tool. It's not a question of gameplay versus graphics. It's a question of graphics versus not-graphics.
It's my understanding that there are free services that perform the function of the virtual tabletop perfectly well, albeit without frills. Heck, I'm fairly sure I could write one myself if I put my mind to it. If I'm going to pay for WotC's virtual tabletop, it's going to have to offer me something substantially beyond what those free services have going on. Graphics quality is a big part of that.
As for performance--I can't believe performance is a serious issue. This stuff isn't going to be animated AFAIK; you're just making a "virtual mini." Nor will there be huge amounts of background stuff going on.
Bottom line, if the virtual tabletop can deliver a really compelling and attractive character portrait/virtual mini system, I'll sign up for it. If not, not. The VT is offering so few features compared to, say, a typical MMO, that there's no excuse to skimp on the features it does offer.