I did this four times.
First time was in Yellow, when I couldn't beat Brock, so i decided to catch a Nidoran and teach it Double Kick. With that i easily pwned Brock. Then i put him in the PC and never used him again.
Second time was in yellow again when i went to Diglet's cave to get a Dugtrio and Defeated Lt. Surge, i put him in the PC immediately afterwards.
Third time was in Ruby when i caught a Geodude solely for the purpose of beating Wattson, (since i picked Treecko as my starter, and none of my pokes could even touch wattson. Knowing, i couldn't train and trade him into a Golem, i kept him in PC.
Fourth and Final time was in Diamond when i Caught a machop just to beat Roark, since i picked Chimchar as my starter, and didn't want to evolve him again. I tried to train machop, but it wasn't working out for me (besides, it was a girl 0_0).