Unfortunately, your opinion is in the minority. There are some cogs that are so popular, and so non-specific, that adding it to core is just only natural. I don’t think imgwelcome should ever be in core, (nor is that being argued here), but a simple customizable welcome cog is something I think a large portion of the community is interested in. I’m pretty sure the v2 welcome cog needed to be turned on anyway, so like many other features Red Core provides, you can optionally enable or disable it.
Yeah, but you have no control over those messages. Server owners want to have a custom message that is tailored to their server. I.E., point to their rules, some instructions, or just to give it their own personality. Even the main Red server uses a welcome cog.