I didn't have one for DD1 due to being in a small flat, my mother was horrifed and insisted she wouldn't learn to walk as they strengthened their legs.
DD1 walked at 12 months.
If you actually look at the way a baby moves, they hurl themselves body first and only their tiptoes are on the floor, I think you can always tell if a newly walking baby has been in a walker as they walk on their tiptoes a lot. Walkers DO NOT strengthen legs as the babys bum and therefore weight is supported by the seat of the walker, their legs are not bearing any weight at all.
DD2 never had one she walked at 11 months.
DD3 well a friend gave me a brand new one, she did enjoy being in it but the minute she started to pull herself up I took her out of it. She walked at 11 months.
DS was in his between 6-9 months and has just started walking at 12 months as well.
I think the danger is if the baby is content in them to leave them in it for long periods of time. I personally know someone who would get their DD up, fed and changed then into the walker all day and she would even feed her lunch and dinner in there, the poor wee thing was only "let out" for nappy changes, but apparently she loved that walker. Took her ages to walk and then it was on tiptoes only.
Personally they are OK I wouldn't spend £40 on one and they take up a huge amount of room and if you can stand your ankles getting clattered into all the time then get one.
Most babies aren't crawling at 7 months, mine were usually 8-9 months and judging by most babies on DSs post natal club, some aren't doing it at 12 months, some are bum shufflers, maybe you have a bum shuffler instead. I wouldn't worry about it at all.