Mainly due to the fact that every sting you have is on an instant cooldown, aside from Viper and Wyvern Sting. Now for Wyvern Sting this may prove to be more useful in pvp situations, especially arena's. But you have to consider what you would be giving up in order to get this talent.
If you pvp all the time, and it's all you do, sure this talent would probably benefit you a little more. But if you plan on raiding or going to instances, I don't see how this could help.
30% Extra damage on Serpent Sting Rank 10 brings it up to a 865 Dot now.
30% Extra damage on Wyvern Sting Rank 4 brings it up to a 1224 Dot now.
Sure that's another 865 or 1224 damage you don't have to do, but wouldn't your immolation trap achieve the same effect? Immolation Trap Rank 6 does 985 damage over time, or with Clever Traps it can do up to 1280 damage. Granted in pve situations you probably won't be using this trap nearly as much as your Freezing Trap, but you won't be using Serpent Sting either, and Wyvern Sting is mostly there for CC.
Assuming you're going for a 0 / 21 / 40 Build, would you really want to give up Mortal Shots for Imp. Stings? If you're going full MM then sure, expirement a little, but just looking at the options and how often I use my stings (aside from Scorpid in PVE and the occasional Viper in pvp, or Serpent against rogues), I just see better places to place those 5 points.
Just my 2 cents~