Yes, this is a good idea, but I don't think BOINC has that feature, at least no other BOINC projects offer it.

But we nearly have that here:
- if you participate in beta testing, this is your top priority project
- 2nd priority is what you choose on the My Projects Page
- 3rd priority you get when you enable "If there is no work available for the project(s) I have selected above, please send me work from another project." on that page

Of course this might lead to more than one 3rd priority project, but currently we have only MCM and ARP, so one of them is 2nd and one is 3rd.

OPN didn't have any work since over a year . But yes, when MAM gets released we would eventually have have a use for such a feature.

The server needs to support it, not the client and I doubt it does, so they would need to write it themselves. And than of course it won't work like a backup project, which sends you only work in case something is idle. Considering that even now when everything runs smoothly, most of my scheduler requests end with "got 0 new tasks", I would just end up with a mix of tasks for all applications just like I'd have them all enabled. Your feature would actually only work as desired on a project with very reliable WU supply, not on a project like WCG with an empty feeder queue most of the time. In the current situation here the difference to enabling all applications will be likely negligible.

If MAM will have similarly large text files like MCM, which are deleted everytime you run out of MAM-work because they refuse to make them "sticky", best solution will be likely (in case you want both) to make sure you always have a mix of both on your computer.