The fact that exactly the same meter without the IS rating is less than half the price would seem to disprove that theory.

http://www.amazon.com/Fluke-True-RMS-Industrial-Digital-Multimeter/dp/B002X9ISWI/

The difference between a $20 and a $200 meter is to do with accuracy and precision, but OP posted this one as an example, and in this case the difference between a $500 and a $1200 meter is the IS rating.