2010: Graphics drivers still vary in quality to the point one would call it "they suck". We need fallback paths whenever the driver or GPU is incapable of handling an operation. Yet, the fallback path can still run into performance problems. In many cases, the effects work well and bring noticeable usability improvements.
I'm not sure I'm understanding the intention behind your comment, even after reading it a few times. In general, the effects do not slow down the user experience, but it's certainly possible in some cases (usually caused by bad drivers). KWin, at least, has fallback pathes, it can use XRender (when a good XRender implementation is available) or fall back to an entirely "2D" rendering, when even XRender does not perform well enough. Others chose llvmpipe as fallback, but that is not a guarantee for good performance. In general, some effects can be offloaded to the CPU, especially for "relatively cheap" effects, that can be a good trade-off to still get usability enhancements, even with bad quality graphics drivers.