As a Forum Resident True Leftist... I really don't see why this should be the case. Sure, it's great to have strong labor unions, but what's the point of forcing membership among people who aren't actually interested in labor activism? If it's only for the money it brings, then we could as well enact a "union tax" and publicly fund the unions.
I think the system used in the Scandinavians and other countries (having welfare benefits be tied to union membership) might make sense though.
What kind of welfare benefits?