Eh, I don't see why Phalanx II shouldn't be equally or more effective than Phalanx. Having merits in it (especially 5/5) eats up very valuable Tier 2 space. If one decides to pursue this route, one should be rewarded in a manner similar to the benefit of having full Slow 2 merits.
Personally, I've never taken a Phalanx II merit, and I have no plans to do so. Like you said, it's less potent than the original version, and if a group really wants me to provide Phalanx, I can RDM/SCH or SCH/RDM to facilitate that.
If there were a significant reason to have it, I might reconsider my decision. Same goes for Enhancing Magic merits. I doubt there are very many mages who actually have this merited; the benefits of meriting other magic types greatly out-weigh Enhancing merits. There's very little point to it as it stands, but that doesn't mean that it has to be that way.
Variety in options is never a bad thing.