Having a spare key hidden under a rock by the back door of your house can be useful too. If you ever get locked out, at least you know you have another way to get in. But, if you lock yourself out of the house once a year, that leaves 364 other days of the year for a stranger or thief to discover your secret key as well.
Remote Assistance and Remote Desktop can be very useful when you need them. But, most of the time you don't. In the meantime, if an attacker somehow finds a way in, or if an attack is created to exploit a vulnerability in the Remote Assistance or Remote Desktop services, your computer is just sitting and waiting to be attacked.