The actual fact is, that during browsing, word processing, spreadsheet operation, audio playing, etc etc, the user applications actually only take fractions of the available CPU clock time which goes at the pace of up to multiple gigaherz per second. If u have a look in taskmanager, u can actually see how much time your browsing really uses. The BOINC or UD science run such, that they take any precious moment, u dont employ a CPU tick. Same time its very good at backing off and let u use the ticks u want. This is true for 99.9% of the time. Only when there is intense RAM to Disk-swapping and file saving with for instance Help Defeat Cancer (can be 750mb), which happens at a higher priority, will u notice BOINC is even running.

The idle time for the user profile is actually a global one in BOINC. If u instructed it to be all time active in the Rosetta or Seti Profile, it goes for all other projects, until u change it at e.g WCG. From than on the WCG preferences will be global for all projects.