question here: how helpful really is getting training from 99-100? I mean, even Restoration that levels slowly, I managed to get my mage to reach during an oblivion gate stuff (using absorb health as offensive spell helped a lot), it did not take that long.
also, as u probably cannot just come to master say "hi, im a 99, make me 100 pls", sou'll have to quest, fight, etc... meaning u'll probably get chances to raise that attribute to 99,5 or something.
master trainers takes you from level 70 to 100 where it's hardest to train for yourself.
You have 5 training sessions for each level up, why not use them. Granted some skill are more useful to get trainers for than others. Has anybody ever used the alchemy master trainer?
Secondary weapon skills can be pretty hard to train, yes hand to hand or a mace is nice against minor enemies and save charges on your enchanted weapon but again hard at high levels.
And yes the purpose of trainers is mainly to get the x5 multipliers, you might want to train magic skills to get access to high level spells even if not primary a mage but here you can make training spells.