When it comes to Roman Units, since they have the same defense value against anything, their nature has absolutely no importance.
The Legates are the most efficient all purpose defense units in the game and they're cheaper and faster to produce than Evocatii, for instance, so it's a better choice overall than would be a necessarily weaker Roman phalanx unit.
Of course, having the possibility to build efficient defense phalanx while using your cavalry queue for something else would be the only relevant reason why such units would be needed.
However, you can still swap queues roles, building Evocatii and Legates at the same time, for instance. Actually, that's what im doing. Evocatii are quite badass units, almost competing with regular cavalry. :p
In addition, balancing them has probably caused the Plarium staff quite some headaches aleardy, noticing they can't be turned into veterans for some reason... Making them quite exceptional.
Beware, please : the fact I'm suggesting to produce Legates instead doesn't mean I'm opposed to adding a new Roman phalanx units anyway.
It would just be a bit hard to balance it so it's not as redundant as all Carthaginian units (except cavalry, and their individual speed) are already, for instance.
To be relevant, it should be tougher than a Thorakites (which is quite difficult already), yet weaker than a mounted Peltast, or it would make them useless or redundant. As you can see, the margin is quite thin.