^ Are you using a controller? Cause I have no problems fitting every spell on my bar as a WHM or AST with the KB/M setup. It's easy to bind everything to an MMO mouse but even without one you can just drag and drop the correct damage spell and heal in a keybound slot in less than 5 seconds as long as you have them all somewhere on the action bar. Not a big issue.
I think people are forgetting the reason behind having leveling roulette in the first place: helping noobs to get story dungeons done. Why can't we queue as a premade? Because then there's a chance that people queuing for specific have to wait longer to be matched with the right roles. Why do our gear and level get synched? So we don't have an unfair advantage over someone who meets the minimum requirements. Why do our abilities get locked? Otherwise we would have access to more utility than low level players and power scaling would be a nightmare to balance.
Things like Medica II, tank stances, Holy, Foe Requiem, Huton, Deathflare and Clemency would be ridiculously overpowered because such abilities are not available to people at low levels. Let's have an example, Deathflare. At level 15, the biggest aoe available to a damage dealer is 50 potency/target. Deathflare is 400. In order to keep things balanced with new players and to not change the level 60 rotation, all spell potencies must be 8 times lower for a max level SMN. Imagine tickling mobs with a 10 potency Ruin or a (total of) 30 potency Bio whilst trying to build up Aetherflow. Not fun. Tank stance could be balanced by increasing damage taken for level synched players, in which case they either have to stay in tank stance and have weaker attacks or stay out of tank stance and take way more damage than a leveling tank. And spells like Medica II can't be balanced at all because aoe heals do not exist at low levels.
As you can see, it's impossible to balance high level and low level abilities without changing how they work (and changing how they work would make this whole request pointless). Leaving the abilities as they are without scaling is not a decent option either because it ruins dungeons for new players and those who enjoy the gradual leveling process on alts. Believe it or not but low level dungeons are not there just to feed veterans easy exp and tomes. They are there to teach people the basics and having the dungeon steamrolled by high level players wouldn't help with that at all. You think players are bad now, trust me they could be a lot worse.