Simply, it depends on who yours will find your SIM card ... if guessed PIN can activate whatsapp
account in his phone, but you receive a message that is whatsapp used on another device. If you only have local communications backups on your phone may hardly read your old messages... but once someone writes to you, can communicate on your behalf
everything provided all contacts, etc. you have stored on your phone not on SIM card...
recommendation is only one, not to lose SIM cards, to 99% if you lose the SIM card - you will lose with the phone