Scholar PCs aren't encouraged in our games, though I have one who focuses on Nature so that I can learn more about the sorcery system. One of our PCs is working on getting a demonic pact and, yes, he's a powergamer.
I don't see the great need to try to screw over PCs with demonic pacts. Corruption can accumulate quickly and is difficult to get rid of, so basically the character is doomed to becoming a NPC at a speed correlated to how much the player tries to abuse the pact. PC above is already at 3-4 Corruption, so if he plays predictably, he'll change his mind about the awesomeness of Master, Aid Me! in 10-12 sessions (at most).
I much rather prefer Vincent's example, where the demonic pact provides ongoing story content without being any sort of disruption to play. Sure, there are plenty of adventure hooks in the other examples, but they are intrusive to a degree. And, I would start invoking the various "screw PC over" scenarios if the PC is abusing the pact, but then, there's an infinite number of ways to screw over a PC. Just people finding out that a PC has a pact is enough to cause the pitchfork and torch crowd to come a calling.
The pact should be fun. If it's not fun for the group, then it shouldn't have been allowed in the first place. Just having it be unpredictable (see how often Elric doesn't get help from Arioch) might make it fun. But, I would spend more time on the demon character and how having a pact can make for more distinct stories and less on the myriad drawbacks to having a pact. The demon should be an interesting character and distinct.
For me, being rather antipowergaming and rather uninterested in typical RPG PCs, I'd find the fun concept for the pact to be by accident or foisted upon my character and for it to be undesirable, that the motivation would be to find a way to dissolve the pact rather than use it. Not quite the Elric situation (I'm sure some well known book has such a relationship) but more similar than the "look at my kule p0wrz" I would guess players are attracted to. Of course, even better would be the delicious irony of the demon actually looking out for the character, truly being more of a guardian angel, but the character having no idea of it and thinking of the demon as something bad to be rid of.