This has been argued quite a lot on the boards, since before 3.5 was released. The search function will help you find some of the old threads.
DR used to be a minimum limitation, like "You must be at least this tall to ride." You were assumed to have the appropriate weapon, and without it you were screwed.
Now it's the other way around. You're assumed
not to have the appropriate weapon, but you can beat the monster anyway. If you do manage to bypass DR though, it gets a lot easier.
For instance, the old pit fiend had DR 30/+3. That was basically meaningless, since anyone strong enough to face a pit fiend would almost surely have a +3 weapon. If you were uncommonly poor and didn't have one, the monster might as well have been immune to your attacks.
The new pit fiend has DR 15/good and silver. It's possible to get through that with your standard weapon; Power Attack helps a lot, but the point is you can beat the monster down by brute force. However, if you happen to have a silver weapon
and a magical effect that gives it the Good property, you can kill it much faster. Visualize it as gaining a bonus, not removing a penalty.
If you really want house rules for DR, you probably want to go
here.