That is minging.
But just as an aside, I couldn't bear the idea of throwing away a perfectly good mattress when I was pregnant with my second son. After all, my first son had only slept on it for about 18 months and we are (always) skint.
I did LOADS of googling and reading about why you should buy a new mattress for every child and couldn't really find any definite reason. Some websites said it was to do with mould spores, some said it was because the mattress would have been moulded into the shape of the other child's body and would be uncomfortable, and the FSID said it wasn't necessary. So I kept the one we had, although I did wash the cover. Think it's just another way of making mums cough up cash to be honest (don't even get me started on the car seats only last for five years business...)