I'm not seeing how Melee Training has anything to do with multiclassing, since what it affects are your Melee Basic Attacks, and not your multi-class powers. It's more about defeating the cognitive dissonance that arises when you have a melee-focused hit-monkey like a rogue or avenger suddenly unable to hit the broadside of an enemy who has turned their back and walked away, or when the party warlord has provided a nice opening.
For a Eladrin Wizard, it allows him to melee fight reasonably with a longsword.
No, it doesn't give him the fancy extra bonus ability that most melee attacks have, but it allows him to melee. The feat allows any PC to melee, hence, it is a multiclass-like feat.
When a foe gets past the front line, I wanted this PCs to be able to melee with that foe in the back lines. I wanted him to Defend when doing this as well, but I could not find a good way to do that yet.
It doesn't matter if the Wizard role is to control. When a foe is in this PC's face, he's going to melee him. That's multiclassing. A Wizard fighting with a sword like a Fighter.
No, that's not 4E game mechanics multiclassing. It's 4e game roleplaying multiclassing. The Wizard acting like a Fighter. Hence the reason I said that this feat shores up the multiclassing rules. It by itself is not "a multiclassing rule", it allows the PC to do a multiclassing-like activity.
I wouldn't doubt that other multiclassing-like feats might not be added to the game in the future. If you think about it, even something as simple as Armor Proficiency Leather is a multiclassing-like feat. It allows a Wizard to armor up without penalty, something only other classes are allowed to do.
Any feat that allows a given PC to do something that only other classes are allowed to do is a multiclassing-like feat.