For me, there is one critical reason to use a font management program:
AUTOMATIC FONT ACTIVATION!!!
And I'm not talking about this bogus "startup" sets crap, I'm talking about the font manager being smart enough to turn fonts on if the file needs them. Currently ATM does this in OS 9 and Suitcase does not (actually, I don't have the latest Suitcase, but my understanding is that it does not do this as effectively or universally as ATM).
As for OS X, to my knowledge (correct me if I'm wrong) fonts are NOT activated on a need basis, but rather they are simply on or off. This is totally lame. I don't want to have 200 fonts on or off al the time. I'm a designer for crying out loud. After looking at Extensis's site, it's unclear as to how the autoactivation feature will be implemented in X.
Right now under OS 9, ATM is capable enough to turn fonts on as they are needed when a file is opened in any application. Additionally, if I decide I want to turn on a new font or fonts, I don't have to quit the application. I just pop over to ATM Deluxe and turn it on.
I have not had the same success with Suitcase (but I don't have Suitcase 10, so I don't know if it's gotten better).
Bottom line, whomever makes the next great font manager, don't make me turn on fonts if the application requests them or points out that they are missing or unavailable. You obviously know they're missing, you just told me, so go find them in my Suitcase and turn them on dumbass! Sheesh. How long is it going to take for these programmers to get this right?
To me, interface issues aside, ATM Deluxe under OS 9 is exactly how a font manager should work. Turn em on when needed and/or allow em to be turned on at any other time without disruption in work. Way cool. Have I said enough different ways?