I'm going to address ONLY the "intercity" type coaches, not transits or "skoolies." The shells used by the premium builders are intercity coaches, such as MCI or Prevost.
The main difference is that the bus shell, power train and running gear were designed to run several million miles (if properly maintained). The bus was also designed for rapid repair (the bus isn't earning if the wheels aren't turning). These mean that you're not likely to wear one out.
The other rigs are a box on a truck chassis -- not as durable, but possibly more cost-effective, if you aren't going to be using it as much.
One other advantage of the conversion is that they have a generally timeless appearance. Most people can't tell the difference between coaches built in 1965 or 2009, while RV styles change every several years.
One DISadvantage is that some campgrounds have a declared "no buses" policy. This is to keep transits and skoolies out -- an intercity coach in good shape is not likely to be kept out.