The vulnerability of Jumps..

[Sax]

Should they also make mages invincible while they cast their 3-second spells that root them in place?

I am no DRG, but as a mage, if I find myself in an AoE midcast I can interupt my spell and move out of range (lag be willing) and recast. I could be wrong, but I don't think this is possible mid jump, so you're basically screwed - to which I say, time your jumps wisely or let me be invincible while I cast 3 second spells!