The only time I truly dislike them is when they are NOTHING ELSE but Darker skin / armor and that's it. At that point, sure, it's lazy, as it adds nothing to a game.
But when it's done well, it's pretty much one of the most enjoyable things out there. When you start perverting the positive qualities of a character into evil in imaginative ways, it can lead to traits and actions that a regular evil character wouldn't generally have. Also to hear the VA / Seiyuu play a role archetype they normally don't get to do is pretty great.
The idea that it's a "Waste of a slot" in most fighters is pretty funny to me, where as most "evil" characters past the original MvC games are pretty unique. Shadow Lady might have been a weird mesh of sprites and effects that never should have been put together, But Violent Ken, Evil Ryu, Saiki in KoF XIII, or even Decapre in Ultra... most of these characters are fleshed out to be very different play-wise, they often get unique animations, art, and voicework, and if they weren't included, you'd be less likely to get another original character, but instead nothing in that "slot" at all.
Basically all of Tekken's expanded cast was an "evil version" of the main character they were connected to at first. Alongside that, Battle Arena Toshinden III decided it's blow up it's character roster by making nearly every playable character have a unique "evil" character based off them. I found this really fun, because we suddenly not only had the expected archetypes filling up a fighter, but now we had African warriors, a Michael Jackson Impersonator, and Jason. It totally jumped the shark and went bonkers, and got Tsukasa Kotobuki to draw a bunch of weird designs that I don't think you'd have seen anywhere else otherwise.
Used right, It's a fun excuse to add to a cast.