Actually, I think that it might indeed be helpful if you read up a little bit on the subject. This question for instance does not really make sense. There are a host of techniques (including microarray technologies) which deal with whole cell changes in mRNA expression, for instance. So, in fact your impression (and thus the premise of the OP is unfortunately wrong).
simplified: DNA is template so it must be pretty inert and stabile, RNA´s are mostly degraded by special molecules after their function is over. Tho its possible and can be done, RNA analysis is quite complex and there is need to "freeze" these molecules that degrade RNA. And when all your cells have allways same DNA, it depends on n variables what kind of RNA´s are produced. (n - very big random number)
Let's not forget that RNA can self splice AND carry hereditary material! It's an enzyme! It's a gene! It's- RNA!!
