I've kind of wondered the same thing. We're doing one of these "combos" to Koh Phangan. On the return, the timings are logical and don't waste any time, but on the outbound I think we have 2-3 hours at the airport before the bus takes us.
I'd certainly be interested in either going for lunch while waiting or taking an earlier bus / ferry even if that meant on our own dime. (ferry is only 1,000 baht and I can't imagine a bus being much at all)...