Try doing few usability studies with client-side software (I'm really talking Windows apps) and you'll see how much pushback you'll get. Consumers just hate installing apps. They absolutely loathe it. Everyone seems to be afraid of viruses and software. Now, this can be overcome to a degree if you're a huge company and have a well-known brand name that people trust and you can also overcome it by targeting a specific segment of the market that are computer knowledgeable and are willing to trust you. I'm not sure but I think that Firefox addons have less push-back than regular apps (just a guess, have no data).

So in short, make sure you do several usability studies with people who fit your market segment.