I think it accomplishes a couple of things which, while minor, I don't see a downside to.
1. The single-stat classes are knocked down a peg compared to MAD classes.
2. Lowers temptation to "compete" with the other players by maxing out their prime stat (I've seen it).
3. Removes the complaint that if someone doesn't start with a capped stat they aren't trying hard enough (haven't seen it personally but read about it here).
4. Lowers "glass cannon" effect on characters slightly by reducing offense and (probably) improving defense.
5. A little less work for me regarding boosting monster stats when building encounters.
Hmmmm, I just never thought, and I'm pretty sure nobody in the groups I've run for really think, that a single primary 20 IS more powerful. It can be, but do the 'single stat classes' (builds really) have some actual advantage? I grant you there are classes that are a little weaker than others, or a lot trickier to build well. I'm just not seeing where that's tied to having a 20 in a stat. For instance there are a dozen ways to make a fighter, including some that will want a 20 STR, but you give up something to get there. Your 20 STR fighter will have less hit points, worse defenses, inferior feat options (this is a big one), and may miss out on a few other things, and will just be a markedly narrower character. He may be better in a theorycraft sort of way, MAYBE, but only when you add up DPR and don't think of any other aspect of the character at all.
Perceptions I can't comment on really. Things like that will vary a lot from table to table anyway. My impression from various groups was that the typical 4e player wants their character to be equally ass-kicking as the other PCs, but most don't calculate that too finely. They always seem to be interested enough in character concept that beyond putting their best stat in their prime and making it credible they might do any other thing that strikes their fancy. I had more trouble making sure people understood that they really did NOT want a 14 in their main stat starting off.
Anyway, I dunno, I think the players will figure out there preferences. I never really saw it as my job to tell them they had to do this or that. Essentials really made things clearer for new players, but I only saw a modest amount of interest from existing players in trying E-classes.