I've got a number of Clarkson style chucks for threaded cutters so never needed to use ER collets with them. However, I do use ER collets with Weldon style cutters with no problem.
From a commercial stance threaded milling cutters are mostly obsolete and plain shank cutters rule. I've never seen a Clarkson style holder in an autochanger, which could be a problem for industry. My Clarkson holders are infrequently used for medium size cutters, 3/4" and upwards.
Andrew