I know this is bordering on off-topic for this site. However, many of the old class guides started with a section explaining why you would pick each subclass, which gave a quite good answer for these kind of questions.
Unfortunately, most of the old handbooks are lost with the WotC forums, and the ones I found for Swordmages leave this part out.
I get Assault Swordmages, they go in a more Striker-y way, increasing damage output. Shielding is also clear, damage reduction is an easy concept, even if it scales quite badly.
Ensnaring teleports the target to you after it hit one of your allies. It would make a lot of sense to yank the archer from the treetop, or the archmage from behind his henchmen to the middle of the melee, but unfortunately the Close Burst 2 distance makes it all but impossible.
So what is the value of teleporting an enemy from a near-adjacent position (Close Burst 2) to an adjacent one? (and you burn your Immediate action on it)