Eyliria asks: "Did the mage change to Invocation not make it in this last PTR patch?"
Ghostcrawler replies: "not sure we're going to change it. Mages might stop evocating ever."
I fail to see his logic there unless raid encounters are going to under 40 seconds in length. The proposed change would have eliminated "prevocating", but mages would still have to cast Evocation to get the buff back up after the initial 40 seconds.
The only reason I can think of is that GC thinks that mages might try to cheese the mechanic through mana gems/arcane torrent/etc. This could be prevented with one small addition. Design it so a mage has to complete an evocation or remain at 100% mana for 5 (10?) seconds in order to receive the buff.