For me, the big problem is the 'likely never heard of' aspect. You wouldn't believe the number of 'credits' I've seen that turn out to be a high school newspaper, or someone's cousin's pet project, or, sadly, flat out don't exist (at least, in Google's eyes).
Telling an editor you are published in a venue they've never heard of is not helpful, because they don't know what the *acceptance* rates are like, never mind the pay. If it turns out 80% of all subs get published at _Mag I don't know about_, that's a red flag. Sure, maybe you've been published in a small mag that is super selective, but the editor is only going to know that if they take the time to find out. In my experience, once you get going, you get tired of looking up every obscure credit that comes along.
It's much more refreshing to see a cover letter that contains only what the editor really needs to know: your name, contact info, and your story's title, genre, and word count.
And the pro credit thing is a whole 'nother conversation, but if you list your pro credits, chances are the reader is going to expect *more* out of your story right off the bat. Not giving any hint as to your credentials either way leaves the reader with a clean slate.
ETA: And nothing I say here applies to querying novels--listing credits there is absolutly important because you are ultimately trying to convince someone to *look* at your work. You don't have to do that with short stories--the story itself will get evaluated no matter what.