Are welcomes a positive for some people? Or is this like starting Slack conversations with "Hey, how's it going?"
They can be positive for me -- if they provide helpful information, explain -- briefly and courteously -- the expected etiquette of the group and set the tone.
That's the key: the tone. Is this group aiming for broad appeal and trying to be welcoming or is it a clique? Forget, for a second, how the users of the forum/group behave -- are the moderators, the sysadmin messaging and the like conveying the tone that is desired for the group? If a "welcoming newbies" is desired, then a welcoming tone should be used throughout messaging.
Unfortunately, being "welcoming" doesn't usually push away the less friendly/welcoming users. Being unwelcoming will certainly attract them and almost every forum site falls victim to it. It usually presents itself the first time you attempt to post something with a full page list of "WE KILL USERS WHO DO ANY OF THE FOLLOWING:", many items in red, often with words that "outsiders" don't recognize so they can't be certain they're violating them. Most posts, then, are from insiders with many "pissing matches" (we get bored, I guess?). Outsiders get trampled on and people complain that "we need better forum software that solves trolls/jerks" :)
I find myself agreeing with you in the respect that "welcome messages" are at least pointless. Fix everything else that isn't user generated, replace moderators with people who have empathy for new users, if "noise" becomes an issue (as it would on larger sites), machine learning coupled with requiring moderation for "first posts"[0] goes a long way. Generally speaking, though, you can't fix dicks. Most of the dicks on the niche forum sites are also the most knowledgeable users and the reason the forum site is so popular ... smaller forums, they're the ones running things. :)
[0] I'm thinking of the bigger problems QA sites get into where new users post the same question, causing hundreds of identical answers because new users often don't figure out how to craft the perfect search query (depending on the kludgy-ness of the forum software) before they figure out the reply/post button and they're usually making that first post because they have a headache already.