It's nice to see them embracing, rather than fighting, third party developers -- though I'd wager it's really an attempt to "control" third party development.
From the FAQ:
"Developers are not permitted to build Custom Clients that are multi-headed or interoperable with any other IM network."
In other words, sure you can build your own AIM client from scratch . . . but if you support any other chat protocols, you've just violated their ToS. So it looks to me like it's an attempt at furthering control, not expanding freedom.