main issue is that in 5v5 you have 34^5 = 45 millions different comps based on spec, and still 161,051 if you just consider class (which means 161k*5 players getting the title ), which makes quite a lot of glads handled out each season
There could however be some kind of "compensation" by spec, the system would have to look the "average" rating of a spec, and give a "bonus-mmr" according to the spec in the composition. Say if system sees there are few highly rated elemental, hpal and spriest, and that moonkin, marksman and unholy dk are really present in high rating, then even if a 2k5-rated elem/hpal/sp looses against a 2k2 moonking/mm/unholy dk, they won't lose much point, and the 2k2 team won't earn much either.
Main problem being how to compute this "spec handicap", given that classes that may be "good" in low-rated match may be "bad" in high-rating matchs, and vice-versa. It also totally ignores any synergies between specs (meaning that if two classes are really good if both are present, having only one might be a disadvantage).