The short answer: Idled hubs are marked as "noindex," so Google will eventually drop them from their index. That means no chance of search engine traffic, and therefore very little chance of income (unless you're amazing at driving referral traffic).
For many hubs, this doesn't matter. If a hub isn't getting any traffic for a couple months and it gets idled... then it wasn't going to get any traffic. Either revise it (in which case it can become un-idled and re-featured) or write it up as a loss and move on.
Where it does matter is seasonal hubs. Let's say you wrote a hub about fixing old radiators. It's going to be popular in late fall through the winter. But who's going to read about radiators in July? If the hub gets idled and de-indexed over the summer, you're then in a position to get no traffic and income come the fall. And that's bad.
Oh, and you'll know it's idled because it won't have the little "H" logo in the "Featured" column on your profile. A quick way to check (if you have a lot of hubs) is to click on the "Featured" column heading and sort by Featured status. That way any idled hubs will be grouped together.