[If your TV had an audio input such that you could connect the center channel pre-amp output to that input on the TV (which it should), you could in theory do this and let the TV's speakers serve as a center channel. Ideally, it would be an input that bypassed the TV's volume control, but if it wasn't (and odds are that it wouldn't be) you could have to adjust the TV volume so that the levels match across the front soundstage.

Would it work as well as or better than what you're doing now? Odds are that it wouldn't - presently, your speakers are helping with the dialogue and the TV's just providing reinforcement. In this alternate approach, the TV's speakers would be assuming the significant burden of most or all of the dialogue as well as a fair share of the on-screen sound effects. That means that you're placing this important burden on the cheapest speakers in the room - the superior freestanding speakers would no longer be able to help with this duty. You could use Pro Logic II's adjustable settings to help keep the front channels in play for that (with a high center width setting), but that wouldn't help with 5.1 sources like most DVD's.