Bore guides...

I don't think they are necessary, as I have cleaned many a rifle without one. But for the money, they seem to make the task of cleaning the barrel, without making a mess, that much easier. Just my .02.
 
I use a Dewey bore guide; it has two 'O' rings that seal off the chamber and action/magazine area from solvents. etc. Also has a tight fitting tube that keeps the cleaning rod from bending (arching) under pressure reducing the chances of scoring the bore.

Most quality bore guides have similar features; I just happen to be pleased with Dewey rods so I bought their bore guide as well.
 
I always use a bore guide, because there are times when entering the throat area uncentered I might run the risk of damage, also to try to eliminate any chance of slapping the lands with the rod if I hit a tight spot. I also bring the jag all the way out the crown and don't want to bring it back in not centered thereby risking nicking the crown. But that's just me.

Btw, I use the Lucas bore guides.
 
If this were JimBob's Gun Forum, I'd say you could get away with using a bore snake which rubs across the side of the crown or chamber and drags crap through the bore time again, and again, and again. But this being "SnipersHide" and assuming you love a precision piece of equipment, then a bore guide is a necessity. Sometimes I have to wonder.
 
I find the bore guides make it easier a lot easier to get my long Dewey rods into the rifling. of my TRG 22 and AIAW In addition it keeps the solvent where it belongs rather than in the action.
 
Are guys really damaging throats, crowns and bores from the brass jag or COATED rod? Brass is much softer than the steel, which is why you use it in the first place, and the rod is coated specifically so it doesn't damage anything?
 
In my opinion, it is not the bronze that has the potential of damaging the throat and lands, it is the core bumping into the end of the throat.
So, if you feel the brush enter the lands and do not push the brush until you are sure the core has entered the rifling, the potential to damage anything is minuscule.

That is not saying that there are not advantages to the bore guides......just that if you use your head they are not absolutely necessary.