My daughter is also on the Read Write Inc scheme, and has moved on to yellow band books (the next up from purple I think) even though her writing is frankly terrible.
They knew she was excelling in reading, and although she knows how to form letters and can phonetically spell words, she has hypermobile fingers so her actual letter formation is no where near the level of her reading. She would probably be on books without words still if they wanted her to progress in tandem!
That said, the yellow band books are quite a leap, with more tricky words not yet covered in the classroom, and sounds not yet covered too (c as an s, g as a j, ea sound, y as an e) so I can imagine the decision was not taken lightly. In my daughters case they went with it I think because its the one thing she is good at and they wanted to boost her self esteem as with everything else she is at the bottom of the class!
I imagine they would not want to put your daughter off reading because there are too many words she cannot decode or recognise in the next set.
Are you doing the questions at the back of the books? I found these really good for extending the life and interest of that band, it also makes sure comprehension is equal to recognition.
In your shoes I would question it further, explain she is bored and you are concerned about her losing interest if not challenged and see what they say.