The disadvantages would be just the time it takes to make, install and maintain them. The inside stem is made wider than normal to facilitate mounting the outside stem. This can sometimes be accomplished by taking a belt sander to the existing inside stem and removing enough material for at least a 5/8th inch wide mounting surface. This of course needs to be done before canvasing
The advantages are -- they add to the strength of the bow, (and stern if you're backing up) which is often subjected to impacts such as: running into rocks, docks, boats, or anything else that a novice canoe-er might happen upon... Yes, and they look pretty nice too.