And why would Illustration of Dragon Soul have more score than it already has? WoW Heroes has chosen item level as the base for gear score. It's not perfect, but at least it makes possible for people to compare each others gear and see their approximate "standings" in gear race. For example, WoW Heroes staff will decide to give higher score to that trinket, because it's awesome. Next day someone else (let's say melee DPS) comes to WoW Heroes and asks to change some other score on site, because that sword he got in Naxx is so much better than Ulduar drops he has seen so far or because his trinket is BiS, though its item level is low.
Even if that doesn't happen, changing gear score just because item is great is not the smartest thing to do in my opinion. I've seen system Wowhead uses and to be honest I'm still scratching my head, every time I use it.
For your gear level and spec Illustration of Dragon Soul is awesome. For some other spec/class it might be just a very good trinket, but not as awesome as for you. Should there be different score for every spec/class?
It's pretty hard to compare different classes and specs, but it's not that hard to compare their gear and enchants, if you take only item level in consideration and not some theorycrafting stuff. Even if some items will get different score, how would you compare a warrior and a mage with same score? And since you mentioned DPS, do you really care that you have 13 less score while using Illustration of Dragon Soul while being at the top of damage meters? I honestly don't care. WoW Heroes doesn't provide any info about player's skill or DPS. It's just that: browsable profiles of most WoW lvl 70+ characters. WoW Heroes doesn't give you advices on how to gear your character nor how to improve your DPS or beat a hard mode as specific class. I honestly don't see any reason why would they want to start changing score system they are already using.