Then what is the point of having them on the same class? You have essentially made 2 different jobs and the only reason you want them together is so that they both get EXP. It really doesn't take long to level a job at all and consider how new jobs are introduced, they start at 20 levels below the expansion max level (excluding HW jobs). This means if you have a job that comes out in an expansion, and you haven't levelled the base class, you would have to start from level 1 and not the much higher level. I really do not see that as a valid reason to have splitting jobs.

The reason Summoners Physick gets so weak is because Physick's healing is based off of the jobs Mind Stat, not the intelligence stat. Obviously, Mind is not a Summoner's strong point, so much weaker heals. As for Vercure, that scales off of Intelligence.