I agree - it's not something that they sit in all day and most of the time it is just for short stints eg. while you have a (4minute) shower, go to the loo, hang out the washing etc.

My DD1 sat in her bumbo from around 5 months as that's when we got it but was able to sit on her own from 6 months so it didn't seem to harm her at all. She still sat in it when she could sit alone as it stops them toppling over and you can use it outside and for feeding etc.

I was recommended by an Early Childhood Development person to use the Bumbo as it supposedly helps them develop the correct sitting position and a straight spine - so I think it is her opinion rather than a fact.