I think the problem is that the unit frames control the layout of the buffs/debuffs, whilst working out how much time remains on them is usually done by a separate AddOn. In other words, in order for another AddOn to add the timers, you'd need to subvert the unit frame's layout code, which is something people obviously don't want to bother with :P
Another problem is that the buffs you see are, internally, nothing but icons. Which means that your Rejuvenation on someone isn't called "Rejuvenation", it's called "Interface\Icons\Spell_Nature_Rejuvenation" (and no, the icon names are not always unique or named after the spell in question -- Lifebloom shares an icon with [Fel Blossom], for example). Also, I haven't examined the code, but I have a nasty feeling that buffs/debuffs can shift around during combat.
To put it simply: adding three extra buttons was a darn sight easier :P
Incidentally, one of the reasons I chose ag_Unitframes was that I can actually change where the buffs/debuffs show up -- currently, I have my extra buttons showing up inside the buff area. Also, having separate buttons allows me to make them larger, which makes reading the time remaining on them a heck of a lot faster. I also plan on doing some wholly unnecessary stuff like having the buttons pulse red when they're about to expire and the target is still taking damage.
In any case, if you don't want extra stuff hogging space on your screen, just cast the HoT when the buff disappears :3
Anyway, sorry it wasn't what you were looking for. Hope you find something
P.S.: I'm a keyboard-jockey as well, but sometimes it's simply faster to zip the mouse over to a recently-expired buff and recast it directly. Plus, you don't even need to use the buttons with this setup.