Obviously there have been some legit reasons given for diving sidemount but I had an interesting discussion with a few dive shop owners on a FAM trip two years ago. They all agreed the biggest reason that SM was being pushed to OW divers was to simply sell more gear.
If properly taken care of, a good BC can last a diver many years. So how do they increase sells to customers they have already sold gear to......teach them a different way to dive that requires new gear and tell them this is the latest and greatest way to dive.
Ok.. so I started with SM because I was doing cavern/cave courses.... now though, I don't even want to dive single back mount. I would rather my SM system, its just so much more comfortable. There is a reason so many people are moving to SM systems.... and you don't even have to dive double tanks... you can single tank side mount... So I disagree with the shops you spoke with.
Tim