You're right in a sense about X/Y, but some idio-...I mean people value them on the level of legendaries. "I HAEV MUDKIPZ - GIEV MII ZEKROM" or something like that.
Of course, I can breed Mudkip Eggs. LOL
I know I used a Treecko in my X challenge (first Champion of the challenge, woot) and won easily, but yeah. If a IV or V challenger, you'll likly be suggesting something else unless that person just happens to have a Mudkip lying around. The guideline is that if it's not catchable, they don't have to try to track it down, and you have to choose something for them that is catchable; that's why, when the challenger after me chose a Gen. V game, I was quick to change to Emolga; I kept it electric and that was that. :) Pikachu isn't necessarily my #1 favourite (it changes), but is one of my top ones, and I just knew it was well covered for the purpose of the challenge. I was ready with the switch. :)