IT IS ROUTINE!! The only way he wouldn't score is if the second baseman threw the damned guy out at home from DP depth!! Geez. Any other possible outcome (DP, force out, etc) results in a run!! Do you watch baseball??
Thunder, while your rationale makes sense in the "common sense" department, the flaw that you're making is that you repeatedly bring up a DP. While we agree that was the intent of the defensive team, a DP cannot be assumed from a scoring perspective. All you can assume is the "first half" of the DP, i.e., the second baseman throwing to the shortstop. (Well, all we can really assume is a FC-the fielder is going to throw the ball somewhere. However, we agree that DP depth indicates the 2Bman intends to throw to the SS). You're not addressing the batter/runner beating the throw to first, or just a simple force out at 2B, which is what the scorer is forced to assume. All the error does, from a scorer's perspective, is prevent the second baseman from throwing to second, first, or home.
I also feel pretty confident that any error on the 1Bman on the second half of an attempted DP would kill the RBI, if for no toher reason than that the 'muff' term in the rules is quite vague. The spirit of the rule is that an error made by the person covering the bag on the latter half of a DP would void an RBI.