You are getting a line of B*llsh*t.
The difference between Dimensions and Optiplex computers are the design of the case(Optiplex is designed to be more rugged and expandable, and the quality of some of the parts(Optiplex has better power supply, better network cards etc). If you are supporting a large number of computers Optiplex might cause fewer service interruptions due to better parts. With better motherboards and parts, an Optiplex might provide better performance than a Dimension, but unless your users are running very demanding applications like CAD,video editing etc., they won't really see a big difference.
There is no way that anything they sell would "damage" the network.
Dimension computers may not be as rugged as Optiplex, and in a high stress environment they may not last as long. But if you are willing to gamble, you could take some of the money you save and invest it into spares.
One of the reasons large corporations prefer business class computers is that the specs for them don't change as often - so if you create HD images for computer deployment, chances are you won't have to change them as often.
James