A quick google shows that you can get turntable-free designs, but they are expensive and the consumer guides seem suspicious of the design -it's done to increase capacity.
http://products.howstuffworks.com/sh...ven-review.htm
Which would imply that turntables are probably needed to heat the food properly in the cheaper models.
That said, I can't see that what you are doing is dangerous, so if you're happy with the quality of the heating of the food, there shouldn't be a problem.