People (resources) should not be overallocated. If ten
projects will each take a week, and they all get
assigned to the same person, they probably won't get
done in a week.
Real situations are complex, but even a simple approach
would be helpful. Dividing a person's effectiveness by
the number of their concurrent projects is
unrealistically optimistic, but much less so than the
current approach, which is to assume that people can do
any number of things at once without slowing down.
I could imagine more complex ways of doing this, but
they can wait. :)