It certainly is a good concern that the simulation might give away the
answer, but this is mainly a problem with many of the simulations on the web
where the author gives the theory as part of the simulation. Knowing the
theory first makes the students less attentive to the results and according
to PER research is probably one of the reasons for low gain.
But the PHET designers have published research that shows that some
simulations produce superior results. For example using circuit simulations
were superior in that students who used them could wire up a real physical
circuit faster than the students who used only physical apparatus. I
incline to the showing the physical apparatus as part of the "concrete
preparation" for using the simulation to get the results.
An example here is the Modeling momentum lab where students are supposed to
do collisions and make a graph of Sum(mv) after collision vs before
collision. Students are not told what to expect from such a graph. Using a
simulation makes the lab go much faster. Then students have more time for
the wrap up where the concept is developed.
Another great use of simulations is where students can be shown what is
going on microscopically, so the PHET gas simulation allows them to develop
the ideal gas equation at the same time that they can see the molecules.
Then they can gain better understanding of why they get the results.
Finally one of the best uses of simulations is where the students can do
labs that are otherwise impossible, very dangerous, or extremely difficult
and lengthy. So I have them develop Newton's general equation of
gravitation and also look at what happens inside of a planet by using a
simulation of a mass where you can put it at arbitrary distances and find
the force on it. They can also put it inside the planet, and they can vary
the mass of the test object and also that of the planet.
I have gotten gain on the FCI/FMCE consistent with an IE course by using
simulations, so I have no proof that they are inferior to physical labs as
learning experiences. Certainly there is the issue of learning how to use
physical equipment to measure things, but that is a separate issue. Anyone
who will be an experimentalist should encounter physical labs. But since
they are only a small minority of students in early physics courses, that is
not a major concern. In either case one can mix a variety of simulations
and physical labs.
A big problem with physical labs is that students tend to mispercieve what
they see and draw unwarrented conclusions. Simulations can be setup so as
to dampen this problem a bit.
The PHET people seem to be convinced that simulations work equally as well
or better than physical labs. They certainly fit into the mindset of a lot
of students. They are "virtual" reality labs, and are believable to many
students.