My nearly 10 month old is still on stage 1... I see no reason to change him as he is quite happy on it. Having said that, I think it depends on your baby, how they get on with their milk etc. There is nothing wrong with toddler milk, follow on milk... It's not really a con per see as it costs the same as first formula and is just as suitable. It's just a matter of personal choice and both follow on milk and first stage milk meets all of the nutritinal needs of babies up to one so go with whatever you want.
I do however agree that not a lot of iron is absorbed from milk anyway though, so it needs to come mostly from solid food whether you use follow on or first milk. Just because a milk contains more iron does not mean all of it will be absorbed.