Off of the top of my head I can think of...
The cooltrainer on the Lapras level of union cave who has all three Kanto starters
The girl on the Blackthorn-New Bark route who has two wartortles
The already mentioned girl on the way to the league who has all three of the bulbasaur line
Ace trainer, post-Rocket Lake of Rage, all three second stage starters of Kanto
Pretty sure that there's at least one more person who uses charmeleon.
And, of course, it isn't really likely to find starters from a different region in the other games, because none of the others are directly next to another region.
So no, they're not quite so rare as it would seem.