Well, the way I look at it is this:
If you just want defense, then why choose Hsien-ko over Haggar, when they do roughly the same thing defensively, but Haggar is overall a better character and you don’t have to spend a bar of meter and go through the hassle of tagging him out to use his assist?
That’s why, IMO, Hsien-ko should be reserved for those ground mobile characters. Haggar will help them, sure, but Hsien-ko offers them something extra.