No, no, no. There's enough to tell you shouldn't get it.

Rain will not hurt a derailleur, as SeattleForest points out. Keep it reasonably clean. And I think if you keep your chain clean and lubed, especially somewhere like San Diego, you shouldn't have trouble for many, many miles.

So, no I wouldn't get a more expensive, less suitable bike just to avoid the derailleur/chain. Look at it this way, would there be so dang many of them around if they weren't practical?