Tears or holes are the number one reason to replace them but when the cartridges have a higher than normal psi rise after a chemical cleaning means that the pores are permanently clogged and replacement should be considered.
I just closed my pool and when I opened up my DE filter I was surprised to find lots of DE still inside. I had not taken the filter apart for 4 years, but I am glad I did this year. I did not see any tears, but I noticed a dark tan color to the previously white colored mesh. Is this something I should be concerned about? I too am wondering if I need to replace the guts of my Pentair FNS 60.