Has nobody read 'till CH 197?! After reading that ch, there's no way that we'll ever see Jellal again! I'm refraining myself from saying anymore in case of spoilers, because it doesn't look like any of you have gotten to the latest ch.
There are several reasons for this. Firstly, if Mashima doesn't bring bring him back, then I would rather he died at the end of the Tower of Heaven arc. Why? Because that would cause a deeper meaning and scar for Erza. Now, he resurrected him, told him to "live and struggle" while he was at the bottom of his sh!tty life, gave him the hope to see a brighter future and then threw him back into prison for the rest of his life while he has practically died one time? Really? That would pretty ruin all the meanings in the OS arc.
Second is that with Jellal's power and knowledge he can always be helpful, maybe he knows something about the Council or stuff like that. Not to mention the fact that Jellal was always handicapped, he has never had the chance to show his true power (in the ToH arc he had to keep the Tower, in the OS he had amnesia/ wounded/ etc), doubt Mashima again would kill him off like that. He can die, true, but not off screened, rotted in prision or executed!!!
Then it's fan service, bringing Jellal would please his fans so it doesn't hurt much, he pretty much has Mystogan's roles now since he can't return anymore. Does bringing Lisanna back have anything to do with the plot (The Dragons, Zeref,...)? No. But Mashima still brings her back for fan service (shippers)!!!
Finally, Mashima just does so because he is his favorite, in his interview he admits that Jellal is his favorite villain, so yeah doing this would please himself too.