Lots of people do this without necessarily being trolls because they feel their comment would be lost, especially if some giant topcomment is dominating the discussion.
People also do the opposite - writing a toplevel comment to discuss (berate, more often than not) multiple other comments - 'I can't believe the nattering nabobs of negativism that have been commenting here, etc'.
In both cases, writing a good, non-repetitive, non-meta toplevel comment is probably better but it's more work and potentially unsatisfying - a lot of the time, a comment like that will in fact be lost in a big discussion.
As I've responded elsewhere in this subthread, a good top-level post can often succeed in gathering enough votes / mod attention to float up.
Probably not on the busiest posts (though those are something of a conversational lost cause already), but on most typical discussions without excessive comments, yes.
People also do the opposite - writing a toplevel comment to discuss (berate, more often than not) multiple other comments - 'I can't believe the nattering nabobs of negativism that have been commenting here, etc'.
In both cases, writing a good, non-repetitive, non-meta toplevel comment is probably better but it's more work and potentially unsatisfying - a lot of the time, a comment like that will in fact be lost in a big discussion.