It's very important.
(first, read this article by Josh Breinlinger - http://acrowdedspace.com/post/47647912203/a-critical-but-ignored-metric-for-marketplaces)
The way you achieve success in a marketplace is by driving liquidity for both your supply & demand.
Demand-side Liquidity = When users come to your marketplace, they can achieve their goals.
Supply-side Liquidity = When supply comes to your marketplace they can achieve their goals... which are almost always to make money.
If you're making a large amount of your supply-side users a full-time income, then you're helping them achieve liquidity.
Now it's not so black and white and it doesn't always have to be a "full-time income." It depends what their goals are.
1) At Airbnb, renters aren't looking to quit their day jobs and become landlords full-time... they're just look to earn a substantial amount of income to offset their rent, mortgage, etc. So in this case, I would probably goal on # of renters that earn >$500 / month... and (in the first 1-5 years) try to grow this number by 10-20% MoM... and maybe by just 5% once you're in the mid-high tens of millions in yearly revenue.
2) At Kickstarter, the goal of the supply-side is to get their project successfully funded. They don't care if the project creator is "full-time"... they just want to make sure they meet their funding goal. This is why they talk about their 44% project success rate all the time - http://www.kickstarter.com/help/stats
3) At Udemy, our instructors want a substantial amount of their income to be driven from their Udemy course earnings... so we look at how many instructors are earning >$2k / month.