When it comes to this subject, two theories have emerged:
1. Ash will leave it at Professor Oak's lab in a couple of months, having done nothing useful.
2. Aipom has a new evolution in Diamond and Pearl and will travel with Ash to Shinou for that reason.
The second one makes more sense (though with no current proof) but I can also see the writers wanting Ash to have one and this being as good a time as any.
I have come up with a new theory that is slightly different, though not without precedent. While evolution is a possibility, this theory goes under the assumption that Game Freak have not given Aipom an evolution in Diamond and Pearl.
Brock's reasoning for bringing Forretress to Hoenn when he leaft behind Onix/Geodude/Crobat was that it had only evolved recently and he wanted to spend more time with it. Forretress had been around for longer than Aipom will have been and I can see Ash doing a similar thing here. (Note that that could still happen and result in a later evolution but I'm taking this further.)
If Ash takes Aipom to Shinou then he has another Pokémon to back up Pikachu - something he regretted not having at the start of Hoenn, particularly in Hoenn Alone. It will continue to be a useful filler in the lineup until Ash has built up a new team. At that point, the plot could then find Aipom returning to Professor Oak. (In Hoenn terms, I'm thinking at about a year in, between Corphish and Torkoal. So it would be similar to Ash's Heracross in that respect.)
I have no idea what is on the writer's minds with the new saga approaching, but I think that is a good guess.
1. Ash will leave it at Professor Oak's lab in a couple of months, having done nothing useful.
2. Aipom has a new evolution in Diamond and Pearl and will travel with Ash to Shinou for that reason.
The second one makes more sense (though with no current proof) but I can also see the writers wanting Ash to have one and this being as good a time as any.
I have come up with a new theory that is slightly different, though not without precedent. While evolution is a possibility, this theory goes under the assumption that Game Freak have not given Aipom an evolution in Diamond and Pearl.
Brock's reasoning for bringing Forretress to Hoenn when he leaft behind Onix/Geodude/Crobat was that it had only evolved recently and he wanted to spend more time with it. Forretress had been around for longer than Aipom will have been and I can see Ash doing a similar thing here. (Note that that could still happen and result in a later evolution but I'm taking this further.)
If Ash takes Aipom to Shinou then he has another Pokémon to back up Pikachu - something he regretted not having at the start of Hoenn, particularly in Hoenn Alone. It will continue to be a useful filler in the lineup until Ash has built up a new team. At that point, the plot could then find Aipom returning to Professor Oak. (In Hoenn terms, I'm thinking at about a year in, between Corphish and Torkoal. So it would be similar to Ash's Heracross in that respect.)
I have no idea what is on the writer's minds with the new saga approaching, but I think that is a good guess.