Hehe, well each to his own, I guess. If you spellweave and pull, just hit the spellweave button again, cause you have it keybound, yeah? It takes less than a half second and you are ready to run. I even spellweave at Entity fight until I see the Aquiring Targets particle effect on me, then I go down and run. But then again some people die over and over again on the Beam even when not spellweaving.
You should NOT run if you pull agro, that makes it harder for the tank to taunt it off you, and the likelyness of another dps pulling it before you can get rid of it to a tank is very high. The key is having tanks that know what to do, and also to use Planar Shift and Subtle Illusion right before you do big nukes, or when you feel that you are going to pull soon, like for example when tank is low on hp and not hitting, trying to change agro to next tank..
A Demo not spellweaving is a waste of a slot in a raid. Yes, I'd go that far to say it is a waste of a slot. You are not maximizing your damage potential. Just look at the Spellweaving stack 6 buffs.
http://imgur.com/ifCyMnS
With stack 6 of spellweaving and then buffing with Hand of God, you can triple the dps of what you would have if not spellweaving. Agro is no problem, just use Planar Shift (7 meter range) or Subtle Illusion (5 meter range) before you nuke hard with Hand of God buffed. If thats a problem too, either use Misdirection or die. That resets agro completely and you won't likely get back in top of agro again for the remainder of the fight.
About Necro vs Demo squishyness, the Demo is far more squishy. I base this upon the reason of Necro has pets that can tank while using Frenzy, Necro has Lifestrike which can crit heal 1500hp+, Necro has Tome of Epithur that can permastun for 15 seconds.