If I were going to the expense of getting a self-threading serger (which is the only way I would go if I needed to buy a new serger!), I would definitely pay extra to get the cover stitch. It is ***incredibly*** useful if you ever need to sew knot clothing -- for example, for grandkid's sleepers, pajamas, t-shirts, knit dresses, pants, etc. If you get out a nice t-shirt from your closet, this is the stitch that is used to hem the bottom of the t-shirt. When you have the cover stitch on a serger that also has differential feed (which I think all sergers have nowadays; back when I bought mine, it was optional and I'm ***really*** glad I went for it), it becomes really easy to hem knit clothing in a flash.
The cover stitch can also be used decoratively on quilts and clothing. Sure, you might not use it every month, but over the lifetime of the serger you are going to find occasions when you are *very* happy you opted to pay for this feature.
Edit: In terms of crafting, the cover stitch would be nice to have if you are making knit clothing for American Girl dolls. Makes a nicer finished look on some clothes than a serged edging.