Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Shizuna]

"The MP cost of the arcanist actions Ruin and Ruin II has been reduced."

Ripped straight from the 2.51 patch notes.

I guess that's something... but come on, really, SE?! That's it?! What about a higher base piety and a shorter enkindle and anything else that would help a bit? =_=