It's also safer. Running executables (with full access to your user account) from random people on the internet is the quickest way to get pwned. Games with built-in mod support often sandbox any code within the mod so that it doesn't have unrestricted access to your machine. (Any game with automatic mod downloading upon server join better be doing this at least.) So it's both easier and safer to try out games (er, mods/maps) on random people's suggestions. (Same goes for browser-based games, since browsers sandbox all code in web pages.)