I think Marten and Faye shippers should actually appreciate Angus more because his relationship with Faye makes the possibility of Marten and Faye ending up together more likely.
Think about it, Angus is Faye's first serious relationship since her dad died (not counting Sven because whatever that was, serious it was not.) That means it's a strong possibility that it will end in a flaming nuclear trainwreck. If Marten and Faye are your OTP, do you really want to have to dig Marten's charred (metaphorical) corpse out from under a radioactive locomotive? Having a relationship with Angus now means that Faye will grow emotionally and be more able to involve herself in something down the road if Things don't work out with Angus.
Now.
I am not a Faye/Marten shipper*. I think that ship has sailed. And I do not think that there is any particular reason for Faye and Angus not to last, but most relationships don't. Dan Savage says that all relationships fail until one doesn't. Maybe Faye and Marten would be comfortable together in a relationship, but I kinda see that as somewhat a fall back position for the two of them. The old "If we're not married by the time we're X years old let's marry each other." situation.
The way I see it now, I beleive it's equally likely that Faye will end up with Angus, Marten, Sven, someone who hasn't been seen yet, or no one at all.
*Because that would mean that Marten won't be available when Dora finally straigtens out her issues and comes running back to him. Also, it will be tough to adjust to a QC universe in which pigs fly, Sara has a fleshed out back story and Hannelore gets married and has a boatload of kids.**
**It was hard coming up with three examples of "impossible things that could never happen" because so many things that were previously believed impossible in QC have actually happened***
***Pizza Girl and Raven returning, for example****
**** It's asterisks all the way down...