I just instituted a coding assignment with our hiring (exceptions made for those with extensive github/similar open source contributions), and so far only have had three candidates do it, but I'm a fan. Mine is crafted so a really competent developer could get it done in 30 minutes, and intermediate 2 hours and a junior probably not finish. I tell them to send what they have in after two hours that I'm as interested…
Are your company especially desirable? Because if not, why should I invest 10-20 hours across ten companies to get one job? Sure I may get lucky and get hired the first place, but thats not very likely, no matter how good I am. To me requiring me to waste that much time indicate the company being a diva, something I wouldn't want in my employer.
10-20 hours invested to find and and sell yourself to the job you'll be spending the next year at (optimistically 1% of the working hours during the next year and realistically a lot less) seems like a good use of time to me, but to each his own.