They aren't using up any cpu power, as long as they are not the currently selected track or are not record enabled. Having them selected, puts them in "live" mode - a state where they are prepared for live input. And that takes up cpu cycles.
Depending on the virtual instrument in question and how it works, it likely will take up some of the available ram.
I was thinking more in terms of a playback situation...where they're obviously ready to play (if there are notes to be played). So I was thinking that if a template had a bunch of virtual instruments and channel strip settings (like the various templates that come with Logic) and one wasn't using most of them, wouldn't it take a load off the CPU if those were deleted?
Fwiw, it might be worth mentioning that in case you use lots of sample based instruments, CPU useage will actually go up a bit (well, it's RAM useage, but in the end the result is less computing power), even if you don't use the instrument, let alone the project will take longer to load. So it's probably not a too great idea to have your templates stuffed with tons of orchestral instrument instances if you only need them occasionally.
Others than that, with Logic you can have a truckload of the most CPU hungry instruments and effects in your templates, all ready to play but not using up any of your precious CPU resources in case you don't actually play back something through them. One of *the* great things with Logic and something almost no other sequencer offers.
I had a situation today where we were trying to bounce down about 20 tracks of various softsynths, including EXS24s , Stylus, and sculpture. No recorded audio tracks. The producer asked us to provide 24bit/96Khz files for importing into Pro Tools. We failed because every time we tried we got system overloads "system overload. The audio engine was not able to process all required data in time." (-10011) .
Even with all tracks muted except the one sound we were trying to bounce. Even trying to bounce the klopgeist click failed. Also tried recording the sound via a bus to an audio track failed. We went back to doing it at 44.1 and that succeeded, forcing us to ask the producer to make the sample rate conversion on his end.
While watching the 4 processors with "system usage" meter- I noticed severe "spiking" on a couple of the processors.(MacPro 2.66x4 with buffer set to 512 in Logic. MOTU 828). Is it that these synths don't like 96K? Other culprits? HELP!!!