Is there any advantage to raising magic skills [sun, vine, ect] beyond level 7?

Only if you decide to change professions (roles) before you've learned the top level spells. Then you can only learn spells at the level of 1/2 of your skill level.

Example: You have a priest who has 7th level in Vinecraft, and can learn Level 7 Vine magic. You make the priest a Paladin. Now the Paladin can continue to learn Spirit magic at the current level, but can only learn Vine spells up to 1/2 the Vinecraft level, rounded down. (7/2 = 3.5 -> 3rd level Vine). Therefore, if you get the Vinecraft level up to 12, your paladin can learn 6th level Vine spells.

-Sazerac

what affect does raising your skill level have on your MP's in that art? if any.

MP's? You mean the ones currently studying the art, I assume...it has no effect. If you are never going to change them from a role that supports that school of magic, it will do them no good to continue adding points there. All the "X"craft skill does is indicate what's the highest level of magic spell you can learn or cast.

-Sazerac

magic points

Duh, boy do I feel dumb! Sorry, Slug! It's been a bit of a day.

I don't think Mana is affected by the skill level; only by the number and type of spells you have in each "school". Wizards and Priests tend to have much higher mana pools than Warriors or Rogues, even after role changes; I've noticed. So I would hazard a hypothesis that it's amount of spells you know in each school, coupled with role, that determines MP.

-Sazerac