I really don't think a lot of thought was put into this weapon. The fact that it's supposed to be part of a set for swordmages, and yet it's way, way better for characters who are not swordmages is a big hint.
Such as Paladins and their Divine Challenge limitations, or possibly work it with their Ardent Strike to have two creatures in a range of 5 squares marked? It would be nice for a Paladin to be adjacent to the creature they challenge, and Ardent Strike another creature over by their ally five squares away.
Fighters and Wardens won't see much use of this, as they want to be adjacent to their marks. Beast Rangers may love the enchantment, but two weapon rangers may prefer other enchantments then two of these. Avengers won't touch this enchantment. Rogues rather like Flanking their targets unless they have a large amount of possible ways to gain CA at range, though it would mean new fear for the "Ranged" rogues who could use Piercing Strike at range 5.
On the other hand, Str-based Clerics can have fun at Range 5 with their attacks rather then just Wis-based Clerics. Bards can use Guiding Strike and War Song Strike at the same distance.
Warlords? Another class other then the Paladin that will love this weapon more then the Swordmage. Commander's Strike usable at 5 squares. Furious Smash becomes a very simple way to give a very needed bonus to any ally, as you can give the +Cha bonus to attack rolls and damage rolls to an ally adjacent to the foe or an ally you are adjacent to 5 squares away. Opening Shove turns into a very useful way to grant an ally free movement as a standard action, if not a decent way to shove a foe around at range and give an ally the attack as well. Viper's Strike and Wolf Pack Tactics? Both also work well at a "Reach" of 5.
For Swordmages? Its still quite a tool to have on hand. Frostwind Blade and Greenflame Blade make the most use of it, while Luring Strike also has a hand in making a Swordmage near impossible to catch. With some of the Swordmage Encounter powers, this really shines. Fox Feint to swap places with a foe 5 squares away, Foesnare to pin a target away from any ally, Transposing Lunge to teleport a target five squares away to a square adjacent to you, and more. Daily-wise? Lashing Asp, so your Reach 5 Melee Attacks also deal +Con poison damage to enemies adjacent to the target you hit. Hunter's Pursuit to be able to constantly immobilize a target out of reach. While the At-Will part may be limited in use, the Encounter and Daily part of having Reach 5 can be quite a reward.
Just because it has synergy with other classes, does not mean it still isn't meant for the Swordmage.
Blasted Enchantment is making me rethink my Human Hybrid Swordmage|Warlock. Hybrid may not even be needed for the keep-far-away approach, though using Dire Radiance on your Marked is still a fun way to drive the DM batty.