8-10hrs is certainly enough time for some sightseeing. If you are unlikely to visit Burma again and there is no way you could squeeze in a night there, yes its worth it even for a day trip. The main attraction is the scenery (do choose a nice spot for viewing) and there is no need to visit every single temple close-up.

Do, however, bear in mind that you will not have a hotel room to rest inbetween your sightseengs. The flight though short is not exactly comfortable and you may not arrive with the best physical form. If you choose to use a horse-cart to move around, the ride itself can also be very uncomfortable (best to use a car under your circumstances). You will miss the sunset scenery which is most spectacular when the temples "glow" against the vast plains. You will also miss the tranquility of Bagan (it is really isolated) at night under the stars, and the cool crisp air at the temples early in the morning.

Do spend a night if you can, but if you absolutely cant its still worth a visit. I believe the TA we used for flight bookings was http://www.radianttours.com/