With Adam's newfound sight, he wants to continue being a lawyer and damn the blind school now. He uproots poor Mary who was very happy being a teacher to the blind, so he can pursue his law career in Walnut Grove of all places. It's really selfish and cruel.
Being a teacher was always Mary's dream and it's what got her through losing her sight a few seasons prior. Now she has to quit her much loved career and follow her newly sighted husband to be his secretary. It's sad that she felt she had to do this.
You could see how upset she was about leaving the blind school and her career. This poor girl has been through so much tragedy in her life, it's a wonder she didn't blow her brains out at some point.
A couple of seasons ago when she thought her sight was coming back, Adam worried about her moving away from him....and ironically, it's exactly what Adam seems to be doing once he got his sight back. I guess all his worry was merely projection.
It was unnecessary to give Adam his sight back imo and only served to make Mary's character all the more tragic. Like she said when she lost her first baby "How much more can I take?!" Apparently a lot because they just kept heaping it on to that poor girl. I think she may very well be the most tragic character ever created for television.