Greetings everyone,
Yes, you're right in that the amount of MP that are required to pull off combos is quite high, but this is due to the fact the effects of these combos (and spells) are incredibly potent. As such, it was decided that they would require a large amount of MP to take full advantage of their added potential.
In regards to the comments that Disciples of War utilize their TP for actions and get waived costs for subsequent combos, this has to do with the fact that magic spells are able to used without first needing to generate TP. This is why this distinction was created.
Since the patch I think players are starting to find additional tricks to mitigate the cost of these spells and I think as time goes on even more avenues will be discovered as players adjust to the change. That said, we will continue to monitor your feedback as we always do and pass it along to the dev. team.