Not true, they patched fallout after DLC's were released. DLC's create problems of their own. So there will need to be patches.
As far as I know, not all version of Fallout 3 were patched, and not all problems was dealt with. The PS3 version is still unpatched and completely broken with the DLC installed (maybe they patched and I did not hear about it, but the last time I checked, it was true).
It is all a matter of economy: if there are enough users and if they think that the patch will sell more copies of the DLC then they work on it, otherwise the resources / man hours are allocated elsewhere.