Flipping the slide allows you to play the sharps with the slide out and the naturals with the slide in. As far as playing along with others, isn't this somewhat limiting?
Hohner and Seydel make harmonicas in B, which would put flat slide harmonicas in C, a real boon; and an Eb would make a nice E, but how often do people play an E harmonica? In fact, you could flip the slide on any harmonica if you had a use for a keyed harmonica a half step higher. But for common Irish keys, like D and G, it would mean the expense of buying separate reedplates to accomplish the same thing. Seydel might accommodate a "retuning," but not (so far) Hohner.
If I were really into Celtic repertoire, with its distinctive ornamentation, I'd certainly look into a flat slide harmonica.
Tom