Depends on what you mean by “temporary farmers”. You won’t be penalized much if any for rebooting or other short-term offline conditions.
If you are online for a while and get data stored in your vault, then go offline for an extended period, your vault will be downranked and become less able to earn safecoin, while being always available and responsive will give your vault a higher ranking and earn you more safecoin.
The earning of safecoin IS the incentive to leave the computer on. My understanding of the vault software is that it will run at a low resource level. No reason to keep a monitor on while not in use, etc.
See Best Safe Node hardware for more data than you can probably use, but a pretty good coverage of some possibilities.
If your desktop is a powerhog, that’s another matter. I’m setting up a couple nodes that consist of low-power Intel Atom processors in netbook and thin client configurations that use very little energy. Others are using Odroid, or even raspberry pi. These can be gotten cheaply if you want to run a dedicated farmer with a low resource footprint.
If your desktop computer is what you want to use for only when you are online, or doing other things (image editing, gaming, web surfing, etc.) you can run a vault on it to access the SAFE network, and turn it off when you’re not using it. It just won’t earn much safecoin in that instance. But maybe it will still earn some, even then.
Hope that helps.