The thing about that appearance is that it's easily handwaveable. Perhaps they were ships kept in a museum which have just been liberated by some people who want to show of the iconic ship from the planets history (as you might see a spitfire flying around at a British celebration)... I was sort of hoping there had been one put into the background of a battle shot, as proof the rebels used at least a couple, and justification for them as an XWing ship.

Well, the Shattered Empire comic(which is canon) pretty much explains and does both of that.

The Theed Royal Guard keeps them in storage.

And while not in active service, Leia flies one on a mission immediately following RotJ.

C'mon gang, use your space-imaginations a little! If those TIEs aren't tethered like in TFA then let's just assume they're held in place with Tractor Beams.

Or Magnets! Space magnets!

looks more like a barge than a ship. I.e. a platform that isnt just a random piece of metal to land on, it has stabilizers and power and probably enough mobility to shuffle around asteroids safely.

I think a lot of folks are over-estimating how many ships we can expect to add based on the new movies. OT, didn't necessarily overflow with ships either. Empire added the Tie Bomber and Slave 1 and that's it so far as I can remember.

That's an interesting point really. (Ships in brackets still lack miniatures)

Off the top of my head, feel free to kick me if I forgot anything:

ANH:

Rebels: XWing, YWing, Falcon, Blockade Runner.

Imps: tie, tie adv. (Sentinel Landing craft)

Scum:?

ESB:

Rebels: transport.

Imps: t-bomber, Slave 1.

Scum: Well it introduced a load of pilots/crew but not much in the way of ships beyond the Slave.

ROTJ:

Rebs: Awing, Bwing,

Imps: TInterceptor, Lambda,

Scum: nope.

Rebels:

Rebs: Ghost, Phantom, (b prototype) (shadow caster)

Imps: TAP, Gozanti, (sentinel class variant)

Scum:? (Gozanti variants, mandolorian fighters, civilian tie fighters, various others)

TFA:

Res: t70, b-transport

FO: t/fo, t/fo/SF, shuttle, lander

Scum: quad jumper

When you look at it like that, tfa isn't doing so bad.

ANH had an YT-2400, specifically the outrider in a background shot.