I am unsure if I am doing this right and I am sure there are many ways to do it.

I have a scene made up of 38 standard terrains of varying sizes and tons of models too.

My scene is something ridiculous like 300,000,000,000 polygons..... may have too many o's! but you get the idea.

I decided to put procedral mapping on a terrain, increased the resolution and then baked to it to a smaller polycount ....about 30,000 polys rather than 1,000,000. The results looked great much better than it looked as a standard terrain but with far less polys. 

 I found the option for decimating polys but the quality seems to get lowered doing it that way. The way I am doing it seems as if I am getting better quality terrain by reducing the polgon count. rendering is quicker and looks better.

Is there any disadvantages doing it this way ?

I am surprised at the results and I am sure there must be a catch ;-)