Before I talk about this, I'd like to let the moderators here know that I'm not complaining about Equestria Girls like I always do. Rather, I'm trying to be rational and explain why Twilight's friends could not accompany her into the mirror, and the way one of the reasons is not that they've degenerated into dead weight, a la the entire non-saiyan cast of Dragon Ball Z. So, if it appears like I'm complaining about Equestria Girls as usual, please don't ban me, because I'm now being rational and explaining a plot point that I have no reason to complain about.

Also, spoilers.

So, Twilight's friends could not enter the mirror with her, because the Elements of Harmony work differently, albeit destructively, in the human world than they do in Equestria. Technically, the human world has no magic, and therefore would not adapt to magic as intense as the Elements, and it's bad enough only one of them, the Element of Magic, was thrown in there. This eventually explains why Sunset Shimmer magically turned into a demon upon putting on the tiara, because a world without magic cannot adapt to magic as strong as at least one Element of Harmony, let alone all six of them.

So technically, Twilight's friends could not enter the mirror because all six entering it would not only disrupt the balance of the other world, but also outright go out of control and destroy it, bringing Twilight and her friends with it. So it's not that Twilight's friends were weak, insignificant dead weight, or the equivalent of the non-saiyans from DBZ. Rather, it was because they were too powerful, and they needed to send somepony with as little power as possible to keep the human world from getting destroyed, hence Twilight alone.

This eventually takes us back to Meghan McCarthy mentioning that Twilight will not outlive her friends, and the theory that Twilight's friends are themselves immortal as a result of their own Elements of Harmony. If Twilight's friends were too powerful to enter the mirror, then this probably implies that they've received their own immortality, despite only Twilight becoming an alicorn princess and not them.

Though, it also doesn't explain why the human counterparts to Twilight's friends became Elements of Harmony themselves. I mean, if they became the human world's own version of the Elements, due to Twilight and the Element of Magic, then that would mean that balance in the human world would've been disrupted, and the other world itself could've been destroyed. Maybe if I were to sum up a guess, it's that those five humans had no exposure to magic like their ponified counterparts, and that the Element of Magic turning them into Elements of Harmony themselves did not use up as much power as all six Elements combined would have, and thus did not destroy the human world. So really, the Element of Magic alone did not have the magical capacity to destroy the other world as all six EoH's combined would, and was therefore able to turn the girls' humanized selves into EoH's without destroying said world.

Once again, I'm not complaining about an aspect I didn't like. I'm just giving a rational explanation as to why all six girls couldn't enter the mirror without inadvertently destroying the other world. And, if I am, then I'm completely sorry for not only that, but also the way I acted irrationally in my previous posts and blogs. I know that acting emotionally over an aspect I did not like would only degenerate me into a whiny, immature brat fanboy, and I promise I'll do my best to keep myself under control.

But overall, yeah. Twilight's friends were not useless at all, or weak and disposable. It's just that they possessed too much power that could have otherwise destroyed the human world had they entered with Twilight. In a way, it's less like DBZ where only saiyans can defeat the villains, and more like the DC Comic universe, where heroes without superpowers like Batman or Green Arrow would be ideal to save the day, just because those with superpowers like Superman and Green Lantern could otherwise inadvertently destroy the world they're supposed to save.