Dire avengers and multiroles.

 Why would you want to take 5 shots that are hitting on 2+ away from your unit?
You should not be making them well-rounded, they are for one task, and one task only. Eldar are designed to control the game, not allowing assault units to assault your squishy troops etc. so if you're not doing this, then you're going to lose.
Your units should always be supported by other units. Your enemies assault units should not be assaulting your shooty troops because in order to do so, your Scorpions, Harlequins, Jetbikes or Banshees should have stooped them or be stopping them. You sould be shooting such units with your avengers, then assaulting them with something else. This is how Eldar works.

There can be many reasons for not taking a dedicated assault unit in your army. Consider your average 1750pt game, you're looking at around 3 troops, the tanks to carry them, outfitting those tanks with weapons, taking more tanks in other slots, your HQs (whom are rarely cheap unless you wish to lose) and finally any elites you may wish. And we all know firedragons are probably the best there and they'll need a tank too! For each squad!

And then once you've built this list and look back at the cost and suddenly realise you're 700pts overbudget and start cutting..

and finally you end up with 3 troops, 2 squads of elites (all in serpents).. maybe 2 fireprisms and a decent HQ.

So... now you start bean-counting. Maximising throughput on every unit. A squad of banshees only works properly against MEQs. Which is fine, assuming you get to charge them in the first place. FDs on the other hand can pop a tank and then you just EML the contents to death. Scorpions have some nice tricks.... buuuttt fall apart once you get to anything more complex than guardsmen. IIRC gaunts rip&tear scorpions like nobodies business so no point there.

So what do you do? Yes it's true that eldar aren't generalists and yes that means that if they start doing things they weren't "designed for" then they start falling apart but consider the following; Would you use assault marines to kill tanks? Do you use wyches to kill terminators? Do you use Trygons to soak up damage? Do you use SM dreadnoughts as gun platforms? What about SM dreads as assault units? Do you ever use Chaos sorceror powers? Gift of chaos?

The answer... is yes. In all cases.

Let us be perfectly honest here, in any game over 750pts the extra TWO attacks from doublecata in shooting isn't going to matter squat in the long term. The last time someone posted about DAs vs terminators, it takes nearly 200 shots to kill 10 terminators. That's more output than a squad of DAs can make in an entire game.

Now what Mortiki is saying (and based off my own experience) is that suppose you have a squad of DAs. Let's say 10. You have an exarch and a PW/SS and find yourself staring down a group of nasty spacemarines. Doesn't even have to be terminators. In my case it was bikers. Anyway, so these marines are going to be able to shoot/charge you. You are going to shoot/charge them. In this instance you might as well shoot&charge. If you skimped on bladestorm and just took defend that's totally acceptable. I mean, not only do the marines suffer casualties from shooting, they strike later than you in assault and only get one attack each.

So WHY take the PW/SS? For a defensive squad. One that is going to bait units into attacking them. The careful art of attack and defend, the parry and riposte. A unit of DAs that gets into a charge can be assumed to have already lost against a dedicated assault unit which is why you try and use your PW/SS exarch to launch attacks against units that aren't dedicated assault units. Like generic marines. The PW/SS exists only to mitigate the damage of a tactical marine sergeants PW. A 33% chance to save against a S4 attack is pretty reasonable, especially since in ideal cases that attack is coming after yours anyway.

So I'll say it again; The PW/SS exists only to mitigate the damage of a tactical marine sergeants PW. If you can get in 2 rounds of shooting before suffering an assault, that's great. If you risk being overrun in the assault, you may as well assault first. If you're facing low numbers of PW attacks, then the PW/SS is an excellent choice and gives you more flexability. You will use this squad to attack enemy troops choices, preferably ones on foot that are already below full strength, as you are trying to force them to fail combat resolution and therefore not be capable of regrouping.

Because honestly? Why would I pay 90+ pts for a dedicated melee (and totally mediocre) elites choice, not to mention their transport, when 30pts on an exarch power and a PW + 33% invulnerable can net me the same result.

Also if you think DAs are good at shooting you are so, so very wrong. They only become good when the 140pt DA squad has a 130pt farseer babysitting them with guide+doom. A PW/SS works on its own.