Fine, she's a cute girl in a cat-suit. We all like those, of course. But Arex was an actual alien-looking alien. Or at least more so than we usually saw amongst Starfleet personnel. That's gotta count for something, right?
I have to admit Arex always creeped me out. The arm coming out of his chect and gauntish features gave me the heebie-jeebies (made me want to donate money to TAS so they could bring back Chekov).