Let's consider this:
You are a wizard with a charisma of 8, who is the minimum level to take Improved Familiar and get an Imp (I use Imp, not because I think most players are lawful evil, but because it is hands down the best creature on the list, and thus makes the best point of comparison). That level is nine, btw, as you don't actually get a feat at 7th or 8th level.
So there you are with a charisma penalty and a character level of nine. That's not exactly the best place to begin when taking Leadership. But you probably have a Great Prestige reputation and a reputation for Special Power (because you are a ninth level PC Wizard). It is also time to get off your butt and summon a familiar. So your choices are: take the Improved Familiar Feat and get an Imp, or Summon up a Raven and take the Leadership Feat.
So what do you get for the Improved Familiar feat? You get a Familiar, who is an Imp. Imps are pretty cool, so that's not all that bad.
So what do you get for Leadership? Your Leadership score is only 9 for the purposes of attracting a cohort (because getting a familiar is dropping your score by 2 for that purpose), but it's 13 for the purpose of attracting followers (because having a house adds 2 to your score for that purpose). So you get one CR 6 cohort, and a CR 2 follower, and ten CR 1 followers. And you get a Raven Familiar.
So your choice is:
1 Imp, Familiar.
OR:
1 Raven, Familiar
1 Erinyes, Cohort
1 Imp, Follower
10 artisans and apprentices, Followers.
How can you look at those two piles and not immediately see that the second pile is bigger? And we haven't even maxxed out the Leadership here. Heck, we are dealing with a character who assigned his lowest priority to Charisma and never increased it with magic. Obviously, if this was a Sorcerer instead of a Wizard it would be even more one sided.
Is there some amazing synnergy between an Imp's abilities and a familiar's benefits that I am totally missing, or is it in fact in all ways superior to have a "normal" Erinyes
and Imp following your orders while your familiar contents itself with exactly the same bonuses to a Raven chasis.
Frank, are you really serious?
Yes. The 3rd edition PHB defines "animal" as including Manticores and any other mobile creature with a nervous system. Unless we capitalize it, "animal" means the
real world definition of "animal".
As to game balance, based on how extremely small the power of an Imp is compared to a 9th level Wizard, is it unbalanced for them to have it for free? Of course not! It's just an Imp. With Lesser Planar Binding they can mass produce those things, and still have a regular familiar which gives them bonuses of some kind.
-Frank