I think one of these reasons is why the sirens didn’t show up:
 
1: They lost their power, when you realize that they are nothing without their singing ability, so they are useless now because they won’t be able to make everyone under their spell. Even though they learnt how to sing as we saw them in “Find The Magic” song but this is just a normal singing people love it, but can’t make them under their spell.
 
2: Or maybe Discord couldn’t summon them because they are trapped in another world, it is known that equestrian magic is floating in the human world, but maybe Discord’s magic won’t work in the human world unless he travells through the portal.
 
3: The sirens might be a great addition to the Legion of Doom, but in the same time it might be a bad decision, the problem is not with Aria and Sonata, Sonata is an idiot and Aria is like she doesn’t care about conquering Equestria, she just follows Adagio’s lead, so they are not a problem to the Legion of Doom, the problem is with Adagio. Adagio is the kind of person who will never follow someone’s lead, if the sirens were there with the Legion of Doom Adagio would also try to betrayal Grogar. Unlike Tirek, Chrysalis and Cozy Glow, Adagio is the smartest of all of them, Adagio will never accept to rule Equestria with other villains, even if the sirens helped the mean 3 to get Grogar’s bell, Adagio might put her own plan to get Equestria for herself, Aria and Sonata are also by her side it will make Adagio’s plans be succeeded by their help because they are actually doing teamwork unlike Tirek, Chrysalis and Cozy who kept fighting with eachother all along even if they decided to work together and that will make everything go worse and the war would be between three sides instead of two.
 
4: Or maybe Discord forgot about them or didn’t hear about them before, who knows?

@PurpleWonderPower  
Agreed. And Adagio is cunning and opportunistic enough that I’m not convinced she couldn’t be dangerous even after losing her powers. Between the amount of magic leaking into the EG world. the relatively high tech level, and the obvious option of using purely psychological methods of manipulation, there’s plenty of ways she could seek her revenge on the Rainooms or attempt to find a new path to power. And as a bonus, if she did wind up using a non-magical approach, it’d definitely catch the girls off-guard, not only because all the threats they’ve previously faced have been magical in nature, but because she herself had been one of those magical threats, so changing up her approach could put the off guard.

They’ve lost all their power, so they wouldn’t be of any us. Although I do wish they would come back at some point. They’re epic villains.

@genervt  
Giving them any sort of power is dangerous. Adagio is not easy to outwit, and her ambitions could easily become a threat to Grogar’s own. Sonata’s an idiot who needs constant supervision. And Aria… I’m not sure about Aria, honestly. Is she only that grumpy because she’s stuck in exile with Adagio and Sonata, or is that just the kind of person she is, and she’d backtalk Grogar just as much?

@Scrounge  
They could still find a magical object (or more) and become a threat again.

Because they’re not particularly useful without their powers, and too dangerous with them.
 
Adagio might be a threat even depowered, as she’s not only cunning and ambitious, but incredibly opportunistic and much better at adapting to changing circumstances than most of G4’s antagonists.

for LUNA SAKE YESSSS

The Dazzlings must return.

The sirens would have been a good choice, but now they’re in the universe next door (and without their hypnotic voices).

Hopefully we will see them again.  
So much potential.

@TarivGy  
Well that one, but the one in the Everfree could be open full time as far as we know.
 
Plus infiltrating Twi’s castle to use the main one isn’t exactly hard.

@Otterhand
 
A classic

@TarivGy  
no. the portal has always been open since RR.

Because all the Sirens can add to the team is internal strife and disharmonious screeching. They already have enough of that.  
Plus, entirely different world with a portal that normally only opens every third moon.

How do we know they didn’t starve to death without their gems?
 
They haven’t appeared since Rainbow Rocks, and either way, they’re powerless.

 
“Papa?”

“She was very vocal about her disdain for the spin-off”

Because they’re banned to another dimension, and (IIRC) have since lost their magic.

Grogar: me and sonata had a problem at taco bell.

If he’s so incredibly powerful and could feed Tirek and resurrect Sombra at full power with ease, fixing/renewing their gems shouldn’t be too great a leap for him. Unless he’s not invulnerable to their songs, of course.

Probably for the same reason you never thought to learn basic grammar.