Some people only have one computer and that must suck. Fortunately I have multiple so I can use some of my alt-miners to do this safely.
People with only one computer can still securely isolate different wallets & apps from each other by using
privilege separation. For example on Linux, run bitcoin/namecoin/i0coin/etc under separate user accounts, and chmod 700 their home directories.
but not everyone can use linux, the use of linux is quite difficult for ordinary people,so they have to find another solution for it