WD 40 will not do any harm to shifters. My 1998 Dura-Ace shifters nearly quit after 4 years. WD 40 totally restored them to good shifting performance and they are working perfectly today. I've known of many cases of WD 40 restoring good performance to other STI shifters.
Right, but its a last ditch repair. You want the factory lubricant to remain in the mechanism as long as possible.