The purpose if IB is to give a class with a primary stat other than STR or DEX an effective basic attack. In the PH, the Wizard and Warlock both were given the ability to make an effective ranged basic attack in spite of not having much incentive to invest in STR or DEX. The mechanism used there was designating an at-will power as a basic attack. Monsters work that way - if they have more than one at-will, one is designated as thier basic attack. There's no reason that same mechanism couldn't have been used with the Swordmage. Just write up a slightly sub-par swordmage at-will and add the special "this power can be used as a melee basic attack." Simple, elegant, consistent, and makes the only way of poaching the INT-based basic melee attack /paragon multiclassing/ (a generally sub-optimal option).
For some unknowable reason (probably a mild mental block of some sort on the part of whoever was assigned the task of writing up an arcane defender), instead of that, the swordmage - and thus, anyone willing to spend one feat on Blade Initiate - got a feat that changed the stat his MBA worked with.
That sets a dangerous precedent for classes or builds (like the 'artful dodger' rogue) who aren't /supposed/ to have particularly good MBAs, and opens up a way for other INT-heavy classes (Wizard, Warlocks, even Tactical Warlords) to get a better MBA than they should have.
Whether that's devestating to game balance, or just mildly inelligant is a matter of opinion.