I'm not understanding why it [the amount of EsoTomes received per dungeon] would need to be reduced. If you are playing on DRG and complete a dungeon you would only get Eso Tomes of Melee Combat and if you were playing on AST you would only get Eso Tomes of Healing, etc etc. Roulette would base the bonus Eso Tomes on which ever job you were on when you queued for the roulette. It wouldn't really be restrictive in any sense besides "If you want to gear up a tank you have to play a tank". Which in turn will only help the community since it would rule out people who play a tank to gear a healer then jump into harder content as the healer with not as much endgame experience as say someone who geared up a healer as a healer. Because sadly some bad players are a result of this. They lack the literal time playing said job because they have geared it up using another job altogether.
I should also note that the Tomes cannot be combined; like I mentioned earlier in one of my first posts that Eso Tomes of Melee Combat would only be able to be used on DRG, NIN, or MNK gear, while Eso Tomes of Healing would only be able to be used on AST, WHM, or SCH gear. So reducing the amount earned doesn't make much sense it would still take 16 weeks to fully gear up a job however with a setup like this players would be able to gear up to 5 jobs completely in the 16 weeks vs the current 80 weeks to gear up 5 jobs (the system we have now). Do keep in mind that means that you have have to run a EX dungeon 12 times to cap ONE type of Tome (roughly 6-8 hours of play time). If you are gearing up 5 jobs then you have to run those dungeons 60 times a week (roughly 30 hours of play time a week << this is a HARDCORE player as many people with management jobs + families don't have 30 hours of free time available a week). So people obtaining a goal like that will be slim; however, it will allow most people to gear up at least two jobs using Tomes under this new proposed method.
So in short: on paper and in practice it would make sense. ;-)