So, why do you need to be 'exemplary' to be a pioneer ? Why are requirements needed in order to make it 'official' or to have the 'title' of 'pioneer'.
Aside from the above responses, it's a shortcut to being judged positively by fellow JWs.
When JWs meet other JWs for the first time, they immediately try to size up and judge the other as quickly as they can. If you can whip out a title like "pioneer", elder", "ministerial servant", etc. the other guy will immediately know what "level" you are on, and accept you that much quicker.
If you don't have such a title, the process takes longer, involving multiple questions ("What congregation do you belong to? How long in the truth? How many elders/pioneers/publishers in your congregation [if you don't immediately know, you're a bit shaky]? When is your next assembly?").
JWs are anxious to appear acceptable to other JWs, and having an official, Society-appointed title is a huge deal to most of them.