Well if you didn't have a "POJO" or two in your J2ME application then your entire application code would be in one file, one class, one MIDlet. Wow, if your application is big enough that would be one maintenance nightmare I would never want to work on.
So, in my drastic answer, the answer is yes use POJOs. Don't create seperate packages, and keep them down to only the bare necessities. But use common sense and logic to determine what to seperate out.