This is true if you overgear the content. If you are running at an appropriate gear level, a tank is a necessity even if only to control where the incoming damage goes.
And it takes almost no effort to overgear any 4 man content barring HM LI, and little enough effort to overgear HM LI.
One can be wearing full Columi (or better) gear in less than a week of hitting 50 without legacy transferring anything simply by doing at least 1 GF run a day, and that already puts you into "overgeared" status for all world Heroic 4s and all T1 HM FPs.
I would think trying to 3DPS something like HM LI with wacky specs like DPS Ops/Smuggs, 3x DPS Sorc/Sage, etc. would make it miserable, but anything easier is basically a non-issue.