My first str8 was a disposable, total investment was four bucks. For me it was a training razor to see if I really wanted to try this. In the end, I was convinced that it str8's were the way to go for me. I suspect others used this same logic.
When I worked at a gun shop I was surprised by how many people couldn't sharpen a knife to save their lives. I was shocked how many of my coworkers didn't know how to sharpen a blade.
From that I would assume many people are intimidated by the idea of honing a razor. So the disposable fills that niche.
I managed to get over a month out of my first blade, with stropping. For S&G's I honed that blade the way I did my others, and was shocked at how much better the results were. Better than out of the box.