The Super Game Boy was great, it let me, for the first time, play Game Boy games on my television via my Super NES. It also, with certain games, allowed for way more colors than the 4 that most Game Boy games supported. I mean, take a look at this Donkey Kong screenshot.
Isn't that great? Extended color palette, custom screen border, and when Pauline gets yoinked away at the end of the stages, she actually calls for help.
Now, if we take a look at the same game in the new-fangled Game Boy Player for the Game Cube
The enhanced palette is gone, the custom border is gone, and Pauline's cries are reduced to a sad little squeak (sounds kind of like a goose blowing a kazoo).
So you might want to keep your Super NES and Super Game Boy around if losing access to those things means anything to you.