I had an indoor soft cover for my Harley CVO ( traded the Harley in on my CB1100EX ). The first winter I used it, amazingly I found dust migrated in under the cover, and actually collected on the bike, trapped under the cover. This was not apparent until I took the cover off in the spring. The next winter, I left the cover off over winter, and in the spring there was little to no dust on my bike.
From that experience, I would only buy an indoor cover as a "bump into protector" in a tight garage space, or if the garage was excessively dusty. I have plenty of space in my garage, and it is always pretty clean ( I keep it that way because I like a clean garage ). My Cb1100 has now spent one winter in the garage, and dust on it in the spring was minimal.
For me, the only reason I would consider a cover, would be if I had to store it outside. Even then, most outdoor covers for bikes and cars caution against leaving a wet cover on for any extended period, as moisture trapped against painted surfaces can fog the finish. With that in mind, if I ever did have to store a bike outside for any extended period, like over a winter, I would probably rent a small storage unit for a few months before I would buy and outside cover. That said, everyone has to do what makes the most sense under their own existing circumstances, and what is within their budget.
