No real reason, I think. If you want to just use that form, feel free to do so. I guess in some cases, "anu seme" is less specific, as you might not be entirely clear what aspect of the sentence you're asking the question about. For example,
"sina wile ala wile toki?" = do you want (or not want) to talk?
"sina wile toki ala toki?" = do you want to talk (or not talk)?
"sina wile toki anu seme?" = ?
In this case, there's not much difference between the two alternatives, but I can imagine sometimes there would be, particularly in longer sentences with various components. The final "anu seme" just illustrates there's some uncertainty about some part of it; the "X ala X" construction more closely identifies this part.
But aside from that subtlety, they're just alternatives, and anyone is free to use whichever one they want, as far as I know.