A little observation I had the other day, last time I was able to play much:
Considering the fact that Weirdfellows have no spells of their own but can cast anybody else's, the not insignificant amounts of effort required to first complete a society's missions, and then become an apostate again to change sides, and that when you join another non-Weirdfellow society, you can no longer use the old spells from the old society anyway (which I have no problem with), it seems maybe a little bit unfair that you then have to erase all the old society's spells before you can learn any from the new society. Especially as this detail isn't mentioned n the Help (or wasn't last i looked).
Obviously you have to be careful to balance things and avoid game-breaking effects that might come from being able to know 2 or 3 sets of spells atonce, but I think it isn't all that easy to achieve anyway, and I'm not sure what other benefits Weirdfellows get from their own missions (wasn't able to complete any before I left originally- which was partly in order to try and complete that mission in fact) compared to everybody else.
If it is a concern, I suppose perhaps there could be some sort of further limits on how many spells total a player can learn, instead? Or maybe other mechanisms could be found for why apostates can't cast their old society's spells outside of Weirdfellows, if the RPing issue is important.