Here's an example of putting something together that allows for a vacation on an RTW that might be cheaper than a normal ticket buying cycle:

Buy a RTW that stops somewhere FROM where tickets are a bargain, then use that ticket to get home, using the second half to pick up the RTW later. Or even better, get that second cheap ticket to somewhere else with a stopover at home (maybe even in both directions?) This obviously requires a lot of research but can be great, especially in the Business or First classes where the ticket price differentials in SE Asia are really striking.

Here's an example. BKK is the cheapest RTW origination in SE Asia (at the moment, unless it's changed recently). You could go: BKK-CMB (Colombo, Sri Lanka), take a vacation along the way or in Colombo. Then, use a CMB ticket from Sri Lanka to the US that allows stopovers in BKK to get back to BKK (this is assuming BKK as a base, it's the same from any Asian hub and if you're not in one you just have to add a segment on a separate ticket). Then you can go to the US on vacation sometime, come back, stopover in BKK AGAIN (depending on airline, this is possible on some ex.CMB tickets), and then next time for a vacation, use the last segment to get back to CMB, and pick up the RTW from there.

You could repeat this from other places, going back home each time, or just do this once and fly most of the rest of the way around the world after that first time, depending on how many vacations you can get in a year and such. But it's one way to structure a RTW for vacations. The pricing will still sometimes be good enough to make it work, but not always, as you guessed. I've now helped about six people with RTW planning, and bought two myself, and in four out of the six cases it was the best option for them, in the other two they were better off with some nested tickets and creativity.