They're not useless, just not nearly as good as they appear on a first glance at the tooltip. Notably, they do not benefit from the 10% stance modifier, and instead of 100/120/135% damage as the tooltip suggests, it's actually 100/114/127%.
There's a superb WeakAuras script on MMO-Champion that will give you pseudo unit frames to help you see whether you're attacking the same target - which is easy to do and difficult to notice. The original thread is here:
Due to the overhead time needed to bring them up to speed (targeting, casting, getting into position, TP, RSK), the clones need to be active for a decent length of time before they make up the DPS you lose by getting them out - I usually figure on about 30s. That means that if they die every 15-20s, it's a DPS loss to keep getting them out.** If they last a little bit longer than that, it's probably not worth the hassle. If they last a lot longer than that, excellent.
The fight I've found them most useful on so far is Council of Elders (although, I note, we have not yet killed them). In particular, on the Troll that charges all over the place - if you stick a clone on him during the Empowered phase, you can completely ignore the damage reflection, since the pet appears to be immune. Depending on how fast you break the Empower, this may or may not be a damage boost (it is for me), but it is a mechanical advantage.
The ability has a lot of pitfalls and design flaws, but it's not useless.
**Note: 30s is an estimate, don't take it as fundamental.