Slayers are not Fighters...
Knights are similar to Fighters...
Thieves... are actually better Rogues than Rogues.
Mages? Not extremely similar to Wizards.
Just out of curiousity, have you actually seen any of these classes in play?
I think the point people are making is that there are differences between them, yes, but that has been true of many other builds as well. Archery rangers vs TWF rangers vs Beastmaster rangers. Str vs Wis clerics. Wildshape vs Caster druids. Etc.
You pointed out how the Orb Wizard and the Enchanter Mage will, despite having access to all the same powers, likely take very different selections because of their various features. But the same is true of the Orb Wizard vs the Wand Wizard. Honestly, of all the Essentials classes, Mage feels closest to the other Wizard builds. The differences are very, very small, and certainly no more than other build variations seen throughout the various power source splatbooks.
They... are also more versatile with the extra At Will Magic Missile for free (in play, MM is fairly devastating when used against a nearly dead foe or a fleeing foe because it always hits)...
Getting MM for free is more potent than some people are willing to admit... There are multiple times to use it in nearly every encounter.
Multiple times to use it in
nearly every encounter? I'm sorry, this really isn't true.
Magic Missile is certainly not useless. On average, it does about half the damage of a normal at-will with maybe twice the chance to hit. So it can be good for finishing fleeing enemies - or it can be bad, if you target an enemy who is just able to survive it. You rarely know enemies HP quite that closely, especially with the small range of damage MM does. (And, as noted, the damage bonuses you mention for it don't actually apply.)
When is Magic Missile actually worthwhile? When the Mage is in a situation where his normal hit chance is less than normal, or when he is weakened. (Auto-hit damage ignores the weakened condition.) So if an enemy is dodging behind walls of heavy cover or the Mage has some huge penalty to hit, or has been weakened by a ghost - those are the times when MM is really nice to have.
But those situations don't regularly come up multiple times in an encounter. I'd say that a Mage
might have it come up in one encounter a day, for one or two rounds of that combat.
Outside of that, the Mage can certainly use it in various situations, but it will generally just be equivalent to using most any other At-Will. The Mage has no massive benefit over the Wizard in this matter. It's a nice little bit of utility, sure, but not a huge difference between the Mage and the Wizard - not in pure power level, nor in how either one actually plays.