I think its no so much that the mattresses "get like that", though i do know that the current recommendation is for a new mattress per child.
That recommendation has changed, but it is a handy excuse if someone is offering you a really awful 2nd hand mattress

http://www.sidsandkids.org/documents...nStatement.pdf
There has been recent media attention in relation to a theory that there may be a link between sudden infant death syndrome (SIDS) and the bacteria Staphylococcus aureus (S. aureus). Recent media reports have been warning parents not to re-use cot mattresses in the belief that S. aureus in foam mattresses is the trigger behind 50% of cot deaths.
SIDS and Kids has reviewed the research to date and has found that there is no evidence to show that there is an increased risk of SIDS for babies who sleep on their back and on a firm, clean, well fitting mattress that is in good condition.