The one on my Benelli M4 has had shells in it (in a safe, mind you!) for probably the past year and a half to 2 years.
The rubber tube DOES eventually lose some elasticity, but it still retains the shells good enough for backwoods hiking and 3-gun type stuff.
The 360 degree shell coverage makes it just about impossible for them to get knocked out, and the tube will keep enough pressure on the shells so bumps and gravity don't cause any problems.
It IS possible to replace them though. Once you take the shell carrier off, there are some screws that attach the back plate to the shell carrier. You remove those screws and you can replace it. I have heard of some guys using a nylon tube to give extra hold on them, without wearing out, but I've never tried it, so I cant really comment.