Most of the time I give it to DPS, I don't see why tank or healer should deserve it more ?

I'll give it to a tank only if he is a really good puller and the healer will get it only if he either, dealt a good chunk of DPS or did an awsome healer job forced by a bad tank or a bad DPS. If someone was nice in chat he might get it reguardless of the class he play and sometimes nobody earn my reccomandation (especialy in PotD with so many bad players making the run a chore).