It literally shaves milliseconds off the casting time for cure. You get maybe 1 extra cure out of the whole duration, 2 maybe with the 'enhancement'. The simple fact is that at higher ranks, there's a significant diminishing return on spell speed, so much so that the effect of POM is so tiny that it makes very little difference overall.
Powerful? We have very different definitions of the term.
As for why I don't tend to use it? Simply put, it has a casting 'time' of its own. Sure it's classed as 'instant' but as anyone who's used Benediction knows, instant is anything but. If I need beefier cures, there's Divine Seal which has the same 'cast time' but guarantees my cures actually work harder for me. A far better payoff than PoM and on a shorter cooldown to boot.
For Holy, maybe it has some small utility. But it's a damned expensive spell, and it's easy to deplete your entire MP bar if you decide to spam it. A holy spamming white mage isn't doing her job of healing the party either, so it's very select in its usefulness.