questions about ON SELECT rules

I'm working on a project where I need to run some commands/functions when SELECT are executed on certain tables. I looked at the rule system and found the following paragraph in the documentation:

  "Currently, there can be only one action in an ON SELECT rule, and it must be an unconditional SELECT action that is INSTEAD. This 

  restriction was required to make rules safe enough to open them for ordinary users, and it restricts ON SELECT rules to act like views."

These restrictions make it impossible  to implement what I need using the rule system. I'm think about tweaking how rule statement is executed to lift some restrictions (more actions, other type actions, etc.) but not sure what impacts this will introduce into the whole system. Why are rules not safe when these restrictions are removed?

Thanks,

Zhaomo