Hmm I don't know. I agree a bit with you that they could make them last longer BUT

In all honesty, it is not hard to rebuff in mid-fight and also having them on a 5min timer does make it interesting if they make it so they actually matter. It makes the fights for the healer/buffer more interesting, a bit like bard buffs in FFXI. On top of it, when you think final fantasy in general, when you are in a boss fight, your buffs doesn't last forever and you need to recast during battle.

It only makes sense that they last only 5min in FFXIV. So I would not mind them keeping it like this, makes for a more interesting gameplay. Not to mention they could make some stats increase the duration. Let's say PIE for instance, so make it a base 3min duration but the more PIE the buffer has the longer the buffs last.