Even if they never post a single answer, somebody asking (reasonable, original) questions is hugely productive to the site... otherwise we'd be sat their looking silly while we chew the cud here on meta.
Re "locked" accounts; actually, many of the (few) people who have been suspended at some point are regular, usually-constructive users. In the case of total trolling/spamming, there are more... "immediate" fixes than suspension.
I don't consider involvement on "meta" a critical measure of their benefit on SO. I fully expect the number of regular meta users to be a tiny fraction of the SO users; that doesn't in any way mean that the people who just stay on SO are less important. For example, I wouldn't expect to see Eric Lippert on meta (although he does have a parked account), but on SO he is a huge asset to the C# fraternity (i.e. pretty much by definition his answers on C# are correct).