While there are many opinions on, and often a gross misunderstanding of, bonding systems and whether or not to bond, the advantages are, with one exception, in my opinion, very clear.
A bonding system mitigates both galvanic and stray current corrosion, in addition to reducing the effects of (not preventing) a lightning strike, while also potentially reducing the likelihood of electrocution.
From a corrosion perspective alone, bonding systems provide a low resistance direct path for stray current back to its source, making it less likely that it will travel through the water, where it is far more likely to rapidly result in severe damage.
The classic example, for stray current corrosion, is a bilge pump wiring connection that is immersed in bilge water. The current from this wire travels though the bilge water, where it reaches a through hull, it exits the boat and re-enters at the prop shaft, which is grounded, and in doing so causes the prop to corrode. If the through hull were bonded, the path back to the battery (electricity always seeks its source, not ground) would be far more direct, which would either cause the bilge pump fuse to blow, or CB trip, or the current flow would be high enough to cause the wire to corrode very rapidly, which would stem the flow of electricity. Without a bonding system this sort of fault can result in a prop being completely destroyed in a matter of days.
The other scenario, for galvanic corrosion, involves protection of underwater metals, some of which may be of question alloy content. If they are borderline, containing too much zinc for instance, a scenario that was discussed on this forum not long ago, then bonding and maintaining anodes will reduce the likelihood of dezincification. Of course such metals should never be knowingly used for raw water applications.
Most lower cost propellers by the way, are manganese bronze, which is a type of brass, rather than bronze. These require cathodic protection, zinc or aluminum anodes, and/or connection to the bonding system via a good quality shaft brush.
The exception involves wooden vessels. Cathodically protected, i.e. bonded, underwater metals generate an alkaline solution which is harmless to fiberglass, however, it does cause delignification in wood, which compromises its integrity. The risk to the wood is essentially too high to otherwise justify the benefits of bonding.
This article covers in detail the advantages and disadvantages of bonding systems
http://http://stevedmarineconsulting.com/bonding-systems-and-corrosion-prevention/
