Why should you care about it. At this moment in time I can't say that you should, I think it is a matter of personal opinion whether or not the fans will like sewaddle. Personally I like it, Ash hasn't had a bug type on his team since heracross and even when he has had one they weren't with him for long. However I understand why you aren't a huge fan, you don't like type redundancies and also it isn't the most creative pick for Ash to have. Though I predict that the writers will go a route that will make us both happy. Since I don't think sewaddle needs much development I don't think it will be used very often and therefore won't cut into the much needed development if Snivy, but every once in a while sewaddle will get thrown in and show off some bug type moves to shake things up a bit.
Also I do think that although it has no major things hindering it now it could have something come up in the future, like trouble mastering an attack or something. Enough time to allow it to give it the chance to develop but nothing too much that would have it eat up a huge amount of screentime.