I kind of made an assumption that a candy bar phone = low-end phone... Based on how the phones with smaller screens generally leave a lot to be desired in terms of web and multimedia experience, features that many (most?) high-end phone owners would consider essential today.
It depends on the application. With the introduction of Qt to Symbian creating an app that runs both on Maemo/MeeGo and Symbian might be easy enough to be worth the trouble, but the hardware of the low-end phones could also be a limiting factor.