On the other hand, you do need fight scenes leading up to the Gatling gun, and you need somebody to be able to kidnap Megumi. Ergo, Gein. And apparently Camo Guy from the Jinchuu Arc as well.
(on a not-unrelated note, I shall be VERY CRANKY if they go the route the manga went and have Kenshin and Sano take Yahiko with them to rescue Megumi, while Kaoru stays behind to heat up the furo and make a complete muck of dinner. Seriouly; the kid has had maybe a month, tops, of kendo instruction at that point; it's the worst example in the entire manga of Watsuki-sensei utterly screwing up the characterization in order to have his author-expy character get to go do cool stuff. The anime fixed it, so hopefully the movie will go with the version that actually makes sense. /soapbox)