I've never tried it, but what you say makes sense. Some reasons I'm not pressed to do it:
- I don't use hop pellets. If I did, and I couldn't keep them out of the primary effectively, I'd probably be much less inclined to pitch into the primary because of all that gross hop goop.
- Pitching into the primary is more convenient. I have an assortment of carboys that act as secondaries, and with all the different types of brews I make, and the sheer volume, sometimes beer sits in the secondary quite a while before a keg is ready or before the beer is done. But I brew once a week, so every week, last week's batch is ready to rack into secondary, leaving me with a primary full of vigorous yeast ready to pitch onto.
I think overall, the *yeast* in the primary is more suited to reuse. It'll be the most vigorous of the two, and you'll probably have a greater viable cell count in the primary than in the secondary. As you point out, though, it's not a perfect world and there are other factors that may make the secondary yeast look appealing. Let us know how it goes if you give it a try