I think it would help refine your clinical skills while still under the watch of a more experienced doctor. But it does exactly what you side, specializes you. That does not mean you can not practice as a general OD, but it does give you the added knowledge to treat things others might not know how to treat.
On the economic side of things...being residency trained makes you more marketable to potential employers.
Well, I think you are more likely to find a better job offer having done a residency. That was definitely the case with me. But that isn't to say you won't find an as good or better job without one. I feel like it puts the odds more in your favor.
The job I was looking for in particular was in an optometry/ophthalmologhy group practice. The residency I chose works particularly close with the ophthalmology residents. I attend all the morning lectures with them, work the same clinics as them and speak at the same confrences as them. Besides the surgical training, I am getting very close to the same residency training as them - for the year I'll be here. That was particularly interesting to the group that hired me.
As far as specialization, I think that you will become much more comfortable with things you may have only spoken of in school, and only did a couple of times at best. In my own case that was true. Two to three day a week I work in a specialty contact lens clinic. It isn't uncommon for us to fit four to five cone patients in a day, actually it's almost always the case. We routinely fit two month old infants with lenses, post-graft patients, Salzman's patients, etc. These are all things that after having spent a year here, I feel very comfortable with. I don't believe that would be the case not having done the residency. I feel like doing a residency was one of the best decisions I have made.