Don't confuse spin bike with spin class. Just because spin classes tend to be shorter and interval-based doesn't mean that you can't do endurance training on a spin bike if you buy one. The only real downsides I see to a spin bike are cost (compared to a trainer) and the fact that some spin bikes may not be able to replicate the riding position you use outdoors on the road. But a spin bike can be more convenient than a trainer if you only have one bike and don't want to switch it between indoor/outdoor duty. Some of the bikes have pretty nice feature sets as well (thinking here about the Peloton cycle, or some of the Cycleops models).
I'm not against stationary bikes. If I could justify the outlay I'd buy a Wattbike in a heartbeat. I wouldn't descibe it as a "spin bike", though, so maybe I'm misunderstanding the OP's question.