I think bulbing is nearly always the best idea. Let me provide you with an example.
Lets say that youre playing the Sidar and popped a great sage. It is late in the game and youve got the Great Library, the Crown, an alchemy lab, a library, an academy, and youre running Caste System so your advantages are skewed heavily towards settling. If you settle him, he could give you 20 beakers a turn (6+1+1+2*(1+.25+.25+.5)).
Alternatively, you can use him to bulb, I dunno, Necromancy, which costs 800 beakers. Note that this beaker cost is about half of what you could get out of a great sage if he were going to bulb a more expensive tech.
Under these circumstances, settling the sage will start to add more beakers than bulbing him after forty turns. Thats a very long time to wait, particularly since youre relatively late in the game, having arcane lore already and all. Plus youll have necromancy to trade.
The situation described above is close to the best possible situation for settling a great person and it only pays off after forty turns.
Obviously whether or not to bulb is very situational. However, if youre attentive to the bulb order you can really come out ahead. Even if you dont know what youre doing with bulbs you can still obtain a significant advantage through bulbing.
FFH2 presents an interesting situation in that great prophets are useful for the altar as well. Of course, it does take 3 prophets to make the altar really useful. Im more likely to want to generate and expend great prophets on the altar than I am to expend other great persons on any given non-bulb activity.
I would like the science bulbing to favour Sorcery instead of its prerequisites. Then, instead of uselessly bulbing Alteration, Divination etc, there would be a great way to get out early mages.
Alteration is an awesome tech. A great sage will bulb Arcane Lore ahead of Alteration and its ilk after you have sorcery. It would be nice to get sorcery a little earlier though through a bulb.