There are some issues I take with Arby's doing this - but it's not for the same reasons that the majority of people here are concerned about. Personally, the "chibi style" art is something that has been used for ages now. It's used as forum avatars, profile pictures, and even just "cute art." Chibi art doesn't mean that they're intending to be sexualized. Next, the usage of "waifu" was done in the typical ironic/tongue-in-cheek method that both Wendy's and Arby's have used in responses or general tweets off and on. Seeing as Arby's and Wendy's have both used "waifu" in comments/tweets in the past, they understand that there's an ironic context to its usage.
Worrying about someone sexualizing Arby's "waifu" is odd, from my perspective. Artists can sexualize anything/anyone if they want to - that isn't the responsibility of the original artist, is it? It would be like people getting upset with Disney for creating characters that are children because some weirdo online decides to make pornographic comics based on them. People will manipulate characters as they please, regardless of the permission of the content creator - that's always been the case, and why parody doujinshi exists (both hentai and non-hentai alike).
My problem is that they're getting free designs from the community without having to pay for it. Granted, they essentially just want their hashtag to trend. Free marketing (#1) and free spec work (#2) are things that I can't necessarily agree with when it comes down to it. Sure, they have been retweeting some artwork but publicity isn't exactly payment (we all should know the "exposure meme" artists deal with by now).