Why? Because the rule serves no real use in the game thats why. Besides as a DM I already control what spells are in the game. A hard cap serves no purpose that simply saying "No, you can't have that spell, how about you make something that does X instead" won't already do.
Now I know some DM's who do not count spells created by the magic user against that limit. Thats not a bad rule but I think % to learn, a rule I do use ,covers that nicely. Can't learn fireball? No problem. Create your own version.
Also a lot of the games that I run have lists and list of extra spells, from Dragon, from the web or just from my own brain.I figure if I don't put a limit on how many spells they can know they might take a chance in something a little less ordinary once in a while and that means for fun for all.
A note on my Spider Cult of Mirkwood Campaign posts - As you may have noticed, I’ve posted a lot about this campaign over the past day or so. The reason: my *Adventures in Middle-earth* campaign is starting up...
1 week ago