To get back specifically to FFP, I think the big thing is that no one wants to see another Portsmouth happening, or worse still, Rangers. It's all well and good saying that City aren't in debt because the money being spent is Mansour's and it's not being borrowed, but what happens when Mansour no longer owns the club?
City will be left with a squad of top class players on high wages (as they would be) bt without the means to pay them with thier own money. If Mansour sold City, and it wasn't another mega rich person who didn't care about making up to £200mil losses each year then City could go bankrupt. There's no two ways around that. The same can be said for Chelsea, though to a lesser extent, they spend far more than they earn.
I do think Roman has been fairly clever though because he's anticipated these changes imo and he set in motion and global marketing policy which on the face of it seems similar to Manchester Uniteds, and why not follow a succesfull model closely. That said, if Roman sold Chelsea tomorrow, Chelsea would be in big trouble, even with no debt at the time of the sale due to Romans generosity, the debt would quickly mount from then on.
The FFP rules are really there to protect the clubs long term, it might mean missing out on some big signings now and then becuase your club just can't justify stretching to buy him for that ammount and believe me, I know how that feels. But when the long term existence of your club is at stake, don't you think its' worth it.