Well, two Dirty Projectors, anyway. The Roots and Jimmy Fallon conscripted Amber Coffman and Haley Dekle last Wednesday to back Mos Def and Talib Kweli on “History,” from Mos’ most recent album. Always happy to see this show try something different. (And to reunite such potent creative partners.) But it’s more than a bit dismaying to see the arrangement here replicate that lousy quality familiar from most DP songs, in which Coffman and Dekle sort of punch in and out as if they were robots (or, you know, figments of Longstreth’s creative imagination). Rarely do a set of vocalists get so consistently typecast as pure, sound-producing instruments–it’s as if Longstreth had lent Questlove his spare xylophone or something. Overreaction? Probably. It’s not like they’re singing against their will. But consider us a bit skeeved out, regardless.