Ironically my ruby starter blaziken is now in emerald while my emerald starter sceptile is now in ruby. Both are level 100 and someday I might trade both of them back to their beginnings. Of course I don't use them anymore.
I usually do keep my starter pokemon, they're some of the best in the game that you can really depend on ^^ It's nice to have them along with the wild pokemon you catch, that way you have one that's perminate, and the other's you can change in and out and you always have one strong pokemon on your side.
Me? I still use my Blaziken, Swampert and Sceptile ^_^ And I love all three of them <3
My Blaziken isn't level 100 yet and I won't use him much but he can kill everything. I beaten a level 100 Gengar, a lv100 Slaking and a lv100 Metagross. He will be level 100 soon.