Years ago I used to ride side-by-side with friends. It seemed we were always trying to talk to each other (not a good idea), and I was usually on the inside line, constantly watching the side of the lane and road edge to watch for problems, cracks, uneven road sections, etc. It wasn't as comfortable as when I was on the road by myself because I was constantly worrying about the other bike.
There are many reasons NOT to ride side-by-side;
1) Two cycles in one lane are wider than one car or truck. It "may" be good for visibility, but it leaves little room for error on either riders part. Why limit your options?
2) You have to depend on your riding buddies capabiilties, attitude, skills, judgement, experience level to maintain a safe gap and control. We have enough to be concerned with our own riding. Why mix in another factor that will impact your riding?
3) If something happens, say a car pulling out turning right into your lane, your options are limited. What if you have to suddenly move over, and you riding buddy wasn't paying attention? He/she has just compromised your options and put you into a further tight spot. As a riding buddy, I would never want to be responsible for my riding partner's possible injury.
4) You might be a great rider, tons of experience, capabilities above average, attention levels above normal, excellant visual processing, but is your riding buddy everything equal and more? Has he/she experienced all you have and possess the same skills?
5) You each react differently to stressfull conditions. You may be handle an emergency condition easliy because of your training and preparedness. Is your riding buddy exactly your equal?
6) What if your buddy suffers a serious brain fart and is suddenly into your lane section or bike?
7) Think of the MSF training where we stressed maintian a safety cushion all around you, or about utilizing your lane position to communicate or increase visibility. Riding close together violates that logic.
8) One of the benefits of group riding is you have extra eyes on the road, you can get info from other rider;s reactions. You can also learn from their riding style, and line through curves, etc. But, you really can't view this or have an opportunity to see what their doing if you are right next to them.
9) If you are constantly concerned about the other bike, then you have lost part of your riding focus. Someday you'll miss something critital that will affect both of you.
10) Riding in a staggered pattern gives the leading rider more room, and the option of indicating hazards to the following riders.
10) Cycle riding is one endeavour that allows us to be totally selfish, our riding is for us alone. Why give up part of the pleasure of YOUR ride, by worrying about the person next to you? And if you aren't worried about them next to you, you should be, because something WILL happen eventually.
So, don't ride side-by-side, especially on any road anywhere near any city. I think the only place I might consider a brief side-by-side is in South Dakota on I-90, or maybe out in the middle of Nowhere Nebraska or Nevada. But, where do you see most side-by-side riders? On in-city interstates, or local backroads in the country, even on city streets (the absolute WORST place for it). If a cycle I don't know pulls up next to me I back off. If a meet a new rider and we decide to ride together I tell him/her I ride a staggered pattern. I have done thousands of miles in group rides with the BMW touring club I'm in. I have learned a lot from my fellow riders and enjoyed RIDING MY RIDE along with a group. Enjoy your ride!