MIDP wasn't designed with smartphones in mind -- it was essentially designed for (somewhat lower-end) mobile phones.
One of the main problems with MIDP is that the user interface API is totally inadequate. The standard lcdui widgets give you almost no flexibility over the look-and-feel, and the look-and-feel they give you is ugly and unprofessional-looking on most devices. For a professional MIDP application, you generally have to implement your own widgets by drawing them onto a blank canvas (which means you have to do a lot of per-manufacturer customization). Plus, the UI is not the only place where the API is like that. It's designed to be simple for the lowest-common-denominator without giving you flexibility for higher-end devices. RIM's proprietary APIs give you easy access to everything the BlackBerry can do.
It's convenient that RIM supports MIDP in case you want to write one simple MIDlet that runs on BB and on other phones. But when programming an app just for BlackBerry, I would recommend going with the RIM APIs every time.