Small indie dev here, talking about this is pretty difficult because of NDAs and stuff, but will try my best to explain.
A bit of context: I'm the cofounder of MixedBag Games, a super small italian studio. We've released Futuridium EP (PC/Mac/iOS) and then Futuridium EP Deluxe (PS4 / PSVita) last year as a two people team, and now we're working on the action adventure forma.8 (PS4/PSVita/Wii U/iOS/PC/Mac/Linux) and on Futuridium VR for Project Morpheus.
MixedBag is now a 5 people studio, so still crazy small considering that we're working on multiple projects and multiple platforms at the same time.
We've been part of the PlayStation and Nintendo indie program since late 2013 and we've been accepted in ID@Xbox in March 2014 (we've applied the first day it was announced, at GDC Europe 2013 if I remember correctly).
First, you can read what Rami Ismail from Vlambeer had to say back in 2013 about the parity clause: it's still pretty much up to date.
http://www.engadget.com/2013/12/04/xbox-one-same-day-launch-clause-nudged-nuclear-throne-to-ps4/
About the clause
Let's put it this way: as far as I know the ID@Xbox contract was never changed and it's still the same we signed (and everyone else signed) from day one, so what was valid from the beginning is still pretty much valid today.
What the clause is about is written all over the internet, and you can still read about it in multiple places. It's very simple: release first on Xbox One, release simultaneously or you can't release the game unless you get a special pass (the 'talk to us' stuff).
Usually it means the game gets approved if you add enough extra content, you can't do a straight port.
Having previous monetary / marketing deals with Sony or Nintendo for timed exclusivity doesn't matter at all, there's no distinguo about that.
What's different with Sony and Nintendo?
Neither Sony nor Nintendo have something like that. You want to release your already released game on PS4 / Vita / Wii U? Go on, you don't have to add anything at all, there are no requirements. A straight port is fine.
If you want to add extra stuff, it's up to you.
Why it's very difficult for a small team to do a simultaneous release
I find it weird it needs to be explained at all, but anyway...
Hitting multiple platforms at the same time requires a LOT of efforts.
We're using Unity for our games, and thanks to the engine and the experience we've matured in the last years we can switch to a new platform and get a game running in a matter of hours. But having the game running is just the very beginning: you need to integrate the platform specific APIs, you need to get the game compliant with all the techinical requirements for each platform, you have to do all the bureaucratic procedures to get the game in Q&A, you need to actually pass Q&A, do (and pay for) age ratings for each platform and stuff like that.
It's HARD. Even for 'simple platform' as Steam it can require weeks of works, and every platform you add just complicate the matter further.
For a five person team like us, with me as the only full time programmer (and I'm also doing game / level design, business / PR stuff... the usual 'small indie team' things), it's simply impossible to guarantee a day one release on Xbox One too.
For forma.8 we're doing PS4 / PSVita / Wii U at launch and it'll already be a crazy amount of work. But we've already released a game on PlayStation platforms and the forma.8 demo on Wii U, so we already have experience with the procedure and we're quite confident that we can pull it off.
Add another console at launch, a console we've never worked on before? It's impossible for us.
Also, adding extra content later is not trivial at all.
My two cents: I really, really think this needs to go, it'll only benefits MS if they get rid of it.
But that's my opinion. It's Microsoft business so if it's fine for them, that's ok: they have their policies, I can disagree but in the end they've probably done their math about it.
Sorry for the long post, hope it helps to better understand what's all the fuss about.