you are right to a point. DEs do help the run but only if they are a run stopping DE like charles grant or will smith etc. guys like julius peppers or shawn merriman that are considered elite DEs are usually pass rushers only
yes we need DEs, but we only have 3 DTs on the roster. 2 of them are actually DTs and the others are DT/DE hybrids
and stoppin the run starts in the middle of the line. yes it helps to have DEs that can contain, but thats more scheme than personel. a solid DT that holds the middle of the line forces runs to the outside where teams become one dimensional. if you know a guy cant run up the middle its a lot easier to stop an offense