7. Having to go into projects that really don't make sense but pay your bills teaches diplomacy and patience.
8. Having long-term financial committments on the one hand (mortgage, school and medical bills etc.) and short-term uncertain revenue is an interesting mental exercise to engage in.
The last one is the killer for me I think, having dabbled in this area. I wonder (genuinely, not sarcastically) how many proponents of consulting are in this boat.
Probably varies a lot by locale btw, if you can rely on state schooling and have a strong social security net it's very different.