I like JOT BECAUSE I can choose to be a warrior with spellcasting if I like. I like the spending of a heroic ability to do this.
I agree there should be some way to facilitate this but getting all the abilities of a Mage after just one session of play is not it.
As noted above, I’d prefer Heroic Abilities be purchased with Advancement Points rather than just sessions played (which means it is a comic out choice to take abilities rather than just length of play) but I have no problem with warriors being spellcasters quickly if that’s what the player wants; some concepts have spellcasting warriors at the get go, so this allows for that.
I’d also note that (in my understanding) the origins of the game are in BRP where everyone could do everything with the appropriate training. If you wanted to learn magic you just sought a teacher; Jack of All Trades keeps that connection where everyone can do everything.