First off, rank 1 spamming when you otherwise wouldn't cast anything is a horrendous waste of regen time with this trinket, and here's why:

To compare regen vs. the +heals from this trinket, I'll convert them both into effective healing done over 1 minute.

This trinket adds the following +heals on a single target, if spell casts are *exactly* 2.5 seconds. Note that using 1.5 second spells is only 13% more effective (math not shown). This also assumes you cast a spell at the very last instant, to refresh the buff duration.

0 to 2.5 seconds: +0
2.5 to 5: +30
5 to 7.5: +60
7.5 to 10: +90
10 to 12.5: +120
12.5 to 30: +150
30 to 120: +0

Average +heal on target: from basic Calculus, average +heal would be the integral of +heals with respect to time, divided by the change in time, or [2.5*(30+60+90+120) + 150*(30-12.5)] heals*sec/(120 sec) = +28.125 [31.875 with 1.5 second spam].

Assuming 4 healers chain cast heal spells, this will give +28.125 healing done / 3.5 seconds (according to Blizzard's formula; not including talents), or ~482.14 healing done / minute, per healer. Using 4 healers, this trinket would give at the most [and this is *very* generous] 1928.57 extra healing done to the main tank, per minute.


What about regen?
I'll assume 1 mana ~~ 6 healing done [that's slighty less than the efficiency of my greater heal]. So, to equal the 1928.57 healing done from the trinket's "use" effect, you'd need 321.43 mana instead.

I can get 321.43 mana from less than 2 regen ticks with raid buffs. If you use even a single, unnecessary heal just to get this stacked, it almost certainly becomes useless.

If the extra heal costs more than 321.43 mana, you're wasting your mana. If the extra heal causes you to break even 2 ticks of regen, you're almost certainly wasting your mana (as a priest [or druid]). Note: for Paladins, this point is much less important; however, 3 unnecessary FoL's would make this trinket useless.