At this point, we have seen:
1 vex raid
1 hive raid
Prison of elders (for all intensive purposes, since skolas is the big baddy, I'll call it a fallen raid)
And another hive/taken raid on the way
With a suspected vex raid following it
But nothing remotely cabal themed
Why?
I believe that the story has been set up as so they have to be more important later, but, as of current, they play little part in the story. The fallen actively attacked us, the hive were returning, and now the hive come for us
(I'm ignoring VoG because, according to some theories, our actions aren't acknowledged of completing it due to the nature of time in the vault...it's a different game theory)
The cabal however are passive. They've set up base on mars, with a supposed failed attempt to go for the moon (see: the cauldron) but haven't gone for us.
So?
My bet is that they'll be important once we've actually talked to the warming Rasputin.
Why?
Because I think Rasputin will have us go to Charlemagne's vault to...do something. Charlemagne was a known warmind, and the canal could possible be attacking it. I think a raid where we have to go rescue the warmind would be equally as cool as one where we have to kill a big boss, no?
[b]tl;dr[/b]
The games story means the canal won't be important until after we've dealt with all current threats. We could bring the fight to them for Charlemagne, but it won't happen until more story with Rasputin.
Because everyone hates Cerebus Vae III and despite the fact that it is one of the easiest and shortest Strikes The Dust Palace also gets a lot of hate. Mars is also the least popular patrol destination. When you combine all these factors it becomes apparent that most Destiny players just don't like having to deal with the Cabal. And until Bungie figures out where they went wrong it with Mars/The Cabal it's best to air on the side of caution and hold back on delivering a major set piece that uses gameplay features that the community will hate and despise.