I dry fire quite often. I usually do it while in my living room. I walk down the street and place a 1" target spot on a light post about 100 yards away, then dry fire at that through my window.
When dry firing, I use a 'Snap Cap' to protect my firing pin from stress cracking. I cannot confirm nor deny (what Gene said above), whether a spent cartridge would suffice, but I'd recommend investing in a snap cap just to be safe.
I can't think of any disadvantages regarding dry firing, except maybe a loss of range time if you dry fire at the range only. There are several advantages however. I don't believe it possible for someone's shooting abilities to decline from too much practice. If anything, it can only help. Dry firing allows you to 'hone' your shooting skills without spending ammo, wear and tear on your barrel and cleaning products. It's quiet and can be done in your home. Most importantly, dry firing allows you to detect any shooting flaws you may have. When firing live ammo, the recoil and the blast of the shot tends to mask any flinching the shooter may be doing. In many cases, this flinching is mistaken as poor ammo or something wrong with the rifle or scope. Dry firing not only lets you verify your bag/rest setup, it allows you to practice your trigger control, follow through, detect flinching and several other things hard ammo would likely conceal.