I won't deny that he's boring but the diss was the hardest diss to be thrown at Em at the time and boring or not, Evidence was and is well respected. Dilated in general was well respected. Em was still very much eating off his underground rep and style at that time, which if he was being true to that image, means it was worthy of response.
Em wasn't in a making tracks for radio play lane yet. Dude was on Sway and King Tech's This or That and featured dudes like Sticky Fingaz and Xzibit on his tracks. He was still getting hype from his Rawkus track. He very much had one foot still in the Underground side of things for his benefit.
Edit: @ J10 I pretty much cover your points with the above. This was Marshall Mathers LP era Em we're talking about. He was not at the level of Nas or Jay in those examples. He was just starting to cement himself.
"Starting to cement himself" is an understatement. SSLP and MMLP have done a combined 16 million units sold (no two Jay or Nas albums have done that). No matter how hard Eminem straddled that fence he was always gonna be pop. His lead singles were always super pop, up until I'm Not Afraid. He was always aware of his status and why he was as famous as he was, and he even took that into account when engaging in feuds.
And just be to clear, mainstream success is not a metric by which we should measure any kind of ability or worthiness, with respect to battling. Just saying there's a clear reason why some people get ignored when trying to feud musically.