laying a brick patio in extreme temperature setting
Weeds grow anywhere...whether you mortar or use sand. 

If you live in a freeze/thaw area, you definitely don't want to use mortar, as it will eventually crack, and cracked mortar is a lot harder to maintain and remove weeds from than sand is.

Also note that you don't want to use just 'fine sand' but polymeric sand. Polymeric sand needs to be wetted after application and it will 'lock itself' into the joints. 

Regardless, every other year or so you will need to likely brush a bit more sand in and pull a few weeds. But that's still a lot less work than maintaining mortar joints.