I think that their point is not that it's a ultimate skin or legendary skin but the theme behind this.
Yes, i like the plusefire cait, she looks so cool. But i don't like that either because it takes from plusefire ezreal.
In my opinion, i don't like that by the same concept "plusefire" have in various champions when they relased plusefire ez, because it was susposed to be unique and it's just as if they didn't had enough ideas for cait and use the plusefire ez as theme.
I have 3 ultimate skins (the DJ sona, Elementalist Lux and Spirit Guard Udyr) and i think that these ultimate skins should be stayed with ther "unique theme" and not be used for another champion's theme.
For example, I wouldn't like to see DJ Bard (JUST AN EXAMPLE), i don't care if it were just a legendary skin or not some cool animations, etc. Is the visual representation between DJ sona and DJ bard. It's just a joke/insult for the wonderful DJ sona skin wich her theme were taken to make DJ bard, making lose the "unique" theme that was made specifically for sona. (specially when i spent money on it, DJ bard is like the cheap version of DJ sona and it's sad)
This is just an example but it has the same feeling that with plusefire ezreal and plusefire caitlyn (In the visual representation and the RP difference that they have)
I'm so sorry if i offended anyone with that, it's just my opinion.
PD: It's ok when a RARE/LEGENDARY SKIN theme has various champions involved; like star guardian, dark star, pool party, arcade, etc. But it's not OK to use an ULTIMATE SKIN that is being used for more champions when they were made to be UNIQUE for a specific champion.