That's more of a consequence than a bonus. It's pretty rare that a caster can't just shift out of melee range.
You're kidding right? Either that or you're subject to soft DMing or playing with three defenders and two leaders.
Spellchuckers will occasionally get pinned down.
Sometimes the spellchuckers need to step up and take some damage. They have surges and hit points for a reason. It's poor resource management to expect certain characters to take basically all the damage. In difficult encounters/days this simply doesn't work. Even if they have the surges to spend there may not be enough triggers so spreading teh damage is imperitive to party survival, let alone success.
Also, being able to set up a credible flank can be quite useful instead of in name only.