Aside from hunting I find they have two real uses. The first, for the right kind of rifle, is home defense. Sure you need a pistol, both to carry outside your home and for those 11pm knocks on the door which are almost surely the next door neighbor wanting to borrow ice or needing help with something. Don't need to scare them half to death with a rifle but you do want to be armed. However if you are reasonably certain there is a person or person's in your home a reasonably compact semiis a great choice. In fact it is very arguably the best choice. The next category is for worst case scenarios where you may need to defend yourself or otherwise project your will past pistol range. That being said rifles have a far more limited role than one might think.
I personally like to take a rifle with me for long trips, both for defense and in case something happens. There are situations where the presence of a semi automatic rifle will make people see the light about their evil ways when a pistol might not.
I hate to tell you this but scenarios where you would openly carry long arms while going about normal visiting, commerce and errands are quite slim. That stuff just doesn't happen much outside fiction novels.
However lets just say something did happen. What, where and how would you be carrying a rifle. One thing that I have seen personally is how convenient fully collapsible (H und K type) or folding buttstocks are. They make the package of a rifle much more compact. One of the real flaws of the M4 in my opinion. Obviously you will need some sort of a sling. There are many types so find one you like and don't pinch too many pennies.
Next comes ammo. Over here in Iraq and Afghanistan we carry 1 mag for our weapon while doing stuff around the base. The thinking is that 1 mag is enough to deal with any immediate threat from direct fire or action. Is that enough for you around the house/ barn area if things get wonky? We have lots of other friends with guns as well as guys on guard and QRF forces that beat what even the biggest survivalist group could realistically put out. Also we all carry the same rifle so I could grab a mag from somebody with their loaded vest on. That is yet another reason to standardize rifles. I would probably be inclined to have 1 extra magazine, either attached to my weapon in some fashion or on my belt. promotes having a small home defense pouch with a reload for your weapon a flashlight and a charged cell phone. To be honest in and around the house if you need more than 2 20-30rd mags either you are back inside in a prepared defensive position, have won or are dead. Sorry but it's true.
If I was taking a walk to the neighbors or the corner of my property maybe a couple extra mags (2-3) wouldn't be a bad idea. The new camelback's have the PALS system of webbing so you could easily stick 2 spare mags on one. If I was going to town, or ye olde barter faire or really anything except some sort of intentional military operation I would feel fine using this sort of setup and having my full out chest rig sitting on the seat of the car or whatever.
I guess the likelihood of contact, speed of backup if applicable and the group you are traveling in would dictate what is reasonable. If I was walking around in Mad Max ville 7 mags doesn't seem like a bad idea but I probably don't need that to walk to the neighbors or around the block.
Really I would probably put more emphasis on being able to carry multiple mags for the pistol, concealed or not, which is far less likely to cause me any problems.