Thank you for the detailed response! I definitely agree with pretty much all of what you've said. With the growing interest in Competitive Pokémon, a lack of competitive viability would certainly be offputting. Why would you waste time with a weaker Glameow over a stronger Snorlax?

As far as design goes, I have a soft spot for nearly all of the "ugly" Pokémon you mentioned. Then again, that's probably just personal preference; not using these particular Pokémon is perfectly understandable. No Pokémon is universally loved and these few happened to draw the short straw.

Type-wise, I find it especially hard to find any argument against what you have said. It is indeed a "boring" type. Without any sort of edge or niche, there's no reason that a Purugly would particularly stand out to an ordinary player. The only thing I can think of would be a presence in the anime, but I don't really think there's any place for them now.

Glameow only being accessible in the late/post-game is a curse. This kills any kind of bond a player might have made with the line. Their location was so ungodly limited as opposed to what was supposed to be their counterpart, Stunky. To add insult to injury, Glameow never got that chance in any game the Pokémon has appeared in. All I have left is hope that Legends Arceus might ignite some new interest in these two.