It really depends on what you compose and how you like to compose.
Most people will not need ot laod EVERY SINGLE SAMPLE all the time, but for some reason people seem to think that is whats needed.
Its not, you could get by with two machines, no problem. Obviously having the tools loaded and composing with the performance tools is inspiring, but its not what you need to compose.
One can compose using the standard articulations and possibly even make "lite" versions so they could be loaded all at once. Composing is different than rendering the performance. You can render the performance AFTER you compose, using the tools, working piece by piece or section by section. It doesn't take that much longer, you just must be able to NOT focus on the details, because VSL does all that for you.
However, yes it is quicker to work with multiple machien, the more you have, the quicker one will work and, it will possibly be more inspiring.
Its honestly all about how you work.
I've heard complaints about having to work with incredible speed and such, but IMO its a bad complaint. Speed comes with usage and practice. Building templates and working with routines is the only way to work with speed. The libraries wont do this for you, its you as a user that must work to make the process faster.
If one belives that a library will make things faster, their not thinking. Set up templates people [:)] Both Mixing and MIDI.