~ From a safety perspective a loud pipe is pretty much useless. The noise is mostly heard next to and behind you, not in front of you. The people that need to notice you for safety reasons are usually in front of you not behind or right next to you. Plenty of ways to increase safety on the road, loud pipes are not one of them.
~ However, If you live in California and lane share at high speed (coming within inches of the car mirrors on either side of you) like I used to, you can scare the crap out of people as you pass them. It may even cause them to swerve just as you've gone by, this may bother other lane sharing bikes crazy enough to follow you, but you should be ok. Having a LOUD pipe while doing this may increase the cager's oh sh1t moment.
If by some reason the cars are making space for you as you approach between lanes, I assure you it has nothing to do with your pipe. Their peripheral vision picks you up in their mirror.
~ So the main reason to get a loud pipe? Because you like the sound and looks, and like to be 'heard' by others, (especially in tunnels
~ When I was 16 I had a Yamaha XT 550 with a loud exhaust and thought I was way cool. Then I grew up, moved to L.A. and had kids. Trying to put my babies down for a nap or to sleep at night....made me rethink some of the choices I made when I was 16...
Want to accessorize your bike? Go for it, but I recommend skipping the loud exhaust.
Want safety? Get a highly visible helmet. Also, Don't drink and ride, wear your gear, msf course etc. etc...