Generating powers... Something i have been doing wrong...

As said, something I had been playing for a while and I was taught by friends who were playing who were taught by the gaming group we were playing at...
Seems odd no one had picked up on this, unless it is a house rule of something that they got you to roll all at once and no one mentioned it.
More strange is that this is also how it was / is played at the local GW store.

TBH if you had 3ML and rolled your first power on divination, get a rubbish spell so trade it for primus, then roll again and get the another one you didn't really want, so try your 3rd for the spell you want then it is your own fault you missed out on the free spell I guess.

Personally I have never been happy with the whole random spells thing in these games. You would have thought that if you are taking your army to war that you would know "This guy here can blow up tanks with lightning with his mind... That guy over there can throw fireballs to burn down woods." In the same way that you wouldn't bring the guy with the lascannon to be the guy in charge of killing loads of grunts while putting the guy with the heavy bolter in charge of taking down a titan.

The trouble is some spells in some decks are clearly trash or fillers. Going in the way back machine for WFB the skaven deck had 13 spells you drew from cards (no rolling there), but in general everyone wanted only a couple of the spells as the rest generally sucked. Throwing a greyseer around the board with skitterleep was bad, however Curse of the Horned Rat what killed enemies and turned them into Skaven was nutty, Plague was also insanely powerful vs Knights.
Same goes in 40k now. People would stack Prescience, Precognition, Invisability, Sacrifice, Incursion, Iron Arm or things like that if they could choose.

Giving a buyable option of Loremaster (like in WFB) might be a good answer as you pay some PTS and you know the whole lore. But make it only available to ML3+