Well nigel, if you're looking for reasons for using a simulator instead of solely real-world testing... for those of us who don't have enough test equipment to do much real test work, a simulator can come in extremely handy. also, when designing things like filters and amplifiers, running a simulation is a much faster way to test out component values than swapping them out in a breadboard.
sure, a simulation is to be taken for what it is, and not assumed to entirely represent real-world behavior... but it gets you in the ballpark, and gives you a good starting point for testing a real circuit... which is good, if you're like me and have to carry all your parts and tools down to the school's engineering building to do any real testing.
with that said, i share your opinion of multisim, sometimes I can't believe they even got that program to market considering how grossly inaccurate and extremely unstable it is... but there are several others which are pretty decent, like switchercad or micro-cap.