I have three reasons why I don't like it:
1. There's literally no point. What does the "artist:" prefix do, exactly? Specially clarify that the username in the tag is the artist, as if it isn't obvious?
2. Unless you would frequently read discussions related to tagging and sourcing, there'd be no way you'd know that it's the way you're supposed to tag artists. Hell, even I didn't know it's preferred until I pointed it out to the media mods in IRC.
3. It'd make searching hard. There are many artists whose work is commonly posted here, but up until recently, they were always tagged without the "artist:" prefix, meaning that the recent images without the prefix would be separated from the ones with the prefix.
I'll show you what I mean:
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