When you're looking for work and need to get a job (e.g. to support a family) spending 10-20 hours working on a single job opportunity is risky because they may just say "sorry, we like you but _____". where _____= {there's a corporate headcount freeze, we don't need that skill, we already have something that does that better, we're not good at hiring}. I still think it's likely a good strategy, just worth acknowledging the inherent risk.
If you have oodles of free time, then sure, it's low risk.
If you have oodles of free time, then sure, it's low risk.