http://www.wowhead.com/item=50345#comments This is an example of trinket that procs "when you deal damage". I just tested it and it gives 2 stacks for each glyphed shadowbolt cast. Note that the stacks are gained when the spell lands (aka when you deal damage), unlike your example of DS trinket which gives it's stacks when the spell has finished it's cast (aka "your harmful spells grant ...") . Why 2 and not 3? I don't know, probably a bug of some sort. But for sure not 1. If blizzard ever releases such similar trinkets, then the glyph will increase your chance to proc.
The RPPM system however does not give higher proc uptime on average, regardless of how frequent you deal damage.
interesting. did you get 2 procs for every single cast? or just occasionally?
All I really know for sure is that the glyph is never a DPS loss, and I think it looks pretty cool. So I use it anyways, and any benefits it may or may not have, however minuscule, are just a happy accident to me.