I'm a level 50 BLM.

I start off the fight in Sastasha NM with a new tank by casting Thunder II then straight into Fire III for full stacks and spam Fire until out of mp, then Bliz III and repeat. The new tank would have no chance and they'd learn nothing about tanking (because I'd be doing it). You'd need to start nerfing Fire as well because it's already pretty powerful at that low a level and a level 50 rotation of it would be rather OP.

SE have implemented the simplest fix, which is to lock the skills, rather than the alternative of having to make a metric ton of adjustments which would serve no purpose.