I donated a trinket from ToT raid .  
Item Link : http://www.db.pandawow.me?item=96385/horridons-last-gasp 
Horridon's Last Gasp proc rate is really low ... you'lll get like 1 proc every 1 min or 2 .. 
Sometimes if you re lucky enough you will get 2-3 proc (stacks) from that trinket  
It stacks up to 5 times so where's point when you get 1 proc every min when duratoin per proc is 10 sec  
Sometimes i get proc from 1 instant heal spell randomly but when i heal a whole raid i cant get that da*n proc or two  
It should proc more than ones per min (Should have more proc rate)