As someone observed: while the GROUP may be able to handle higher-CR / higher-EL encounters, the individual risks and challenges increse; the chance that SOMEONE among the party-of-four-and-a-sidekick will die goes up out of proportion to any changes in XP awards.
Now, someone said a party of 4, with 1 cohort, would face creatures of CR11 or CR 12; they aslso stated a party-of-four and four cohorts might face encounters twice as tough as the same party with no cohots.
Well, I hate to tell you, but EL 12 is "twice as hard" as EL10 (a matched pair of CR 10 baddies is EL 12). EL 11, by extension, is probably 150% as hard as EL10.
One extra, guaranteed-to-be-lower-in-level character, and a 50% increase in challenge levels? THAT hardly seems fair, either!