Well when you think reasonably about it, popping techs from huts offers the best hammers-to-beakers conversion rate you're going to get. Figure an explorer costs 40 hammers; in a heroic-epic city with a forge you'll get +125% on that, so it's effectively under 18 hammers. In turn, a typical explorer probably pops a hut every 15 turns, and maybe 1/4 huts give a tech. About 800 beakers per tech, so it's worth 13.3 beakers per turn. Considering that you could build 4 explorers for the cost of one library, it's clear that the opportunity-cost of building a library is 53.2 beakers per turn, so you'd need over 200 base commerce in the city for the library to be worthwhile in comparison.
I think the numbers speak for themselves; it's clear that spamming explorers is a more effective research strategy. Of course, this doesn't consider the true pro play, which is spamming the cheaper scouts.