I had been chatting with another Second Life resident earlier today, and offered him an invite to the 4Freedom group. He declined the offer, because he was at his limit on groups. After being told that, I started thinking "Why did they choose that limit?" Then, a pair of theories dawned on me. Either by flawed testing or a flawed chat system (or both) they felt that their servers could not handle the lag from users having more than 25 groups.
We all know that the system is flawed, because it frequently drops messages, and lately group chat sessions have been crashing a lot, the Free Scripting Library especially. And as for flawed testing? I think the Lindens must have been testing the group chat lag with a "worst case scenario". Perhaps this would be done by having all of the testing groups sending and receiving chat messages continuously. A valid test for seeing worse-case load, but not a valid test for everyday load.
For me, the only groups I get a lot of chat from are Free Script Library and Isle of Wyrms Citizens.