If you take absolutely no steps to secure your web server you are asking for trouble - even small precautions on your behalf can make a very big difference to the safety of your machine.
If you are an ISP looking to attract more users to hosting with you, do not think installing PHP is going to be an easy ride - using safe mode is just the beginning! Be sure to place very tight locks on the amount of resources each user on your server is allowed to chew up, and make sure the permissions on the machines are as tight as they can possibly be. Do not rely on safe mode to do your job for you - the PHP developers do not encourage safe mode for use as a standalone security measure.
Encryption is no easy task to perform, but the result is that you have very secure data, which is gold dust on the Internet. Yes, proper encryption is slow, but it is crucial that you do not try to take any shortcuts when it comes to the security of your data.