If they reaffirm/commit_to Sailfish being 100% based on MeR/Nemo in all but the uppermost layers*, & clearly define that linkage, then it's not a big deal.
But if they continue to remain vague on that front, right up to release day, then I'm going to start on the kittens again...

*except for proprietary stacks like AlienDalvik ofc

i've just looked at the nemo status matrix and for the n900 it can make calls and send/receive sms. so..... maybe i'll install it and start contributing. thanks for the steer.

Yeah but they've only gone this way because the Sailfish_Alliance (remember that, WTF ever happened to it!?) has taken so freaking long to get off the ground.
If they had stopped "fart-arsing" around MONTHS ago, both projects could be working on the same UI/UX now, or very soon...

Just the UI? It's a big waste of resources for Jolla's Sailfish & Nemo, but especially Nemo, the entire community could be working with common components & UI/UX.
That way there's no messing around with porting apps between 2x platforms that are essentially supposed to be helping one another, not hindering one another in some way.
But as you said, so long as most of the important middle-ware remains exactly the same it shouldn't be a huge problem, but it's far from optimal.

That's why they were originally going to keep only "core/stock apps" closed, & license that version of Sailfish...
If Silica itself is completely open, then why the heck not use it as the basis for Nemo's UI/UX, instead of building completely new/separate components?

I don't agree at all, Sailfish should've been opened-up ages ago, or shortly after the 1st phone is released at the absolute latest.
It makes more sense that both projects are merged & have essentially the same code-base....
Jolla can still keep it's originally touted "commercial" flavour of Sailfish, a version that has mostly closed apps (including some closed ones from 3rd-parties).
But the rest should be left for a totally open flavour of Sailfish, one that's administered exactly like Nemo has been for ages now.
Or the code should be handed over to the Nemo project, & mostly/totally supplant the current Nemo code...
Which will (predictably) turn into a political ****-storm, and it never had to, if only they'd pulled their head out of their arse earlier.

As I understand from what you all are writing the thing stops looking that good as it did for me in the past

It's never as good as the fanboys says. But it pretty damn good anyway because it's a new Os. All new OSes provided to us(the customers) the are pretty damn good in their own ways. Fear not!

A bit of rationale about why we've decided to go with new Nemo UI and seperate components:

UI is not something that can be done via commitee, well, it can, but the result will not be good. The Sailfish UI has a certain target and is designed by the Jolla chief designer. The only thing they could do was to throw code over the wall. This is not engaging for the community to create anything. It would lead to something like android, where all the community involvement is just cooking ROMs with different set of included apps.

We want to take nemo somewhere else. We don't want to move from a cheap Harmattan clone to a cheap Sailfish clone. It's GUI is a community effort and we want to experiment and do things that a commercial company wouldn't do for fear of general acceptance/market performance.

The hard part is not creating the gui for the apps, it's the logic behind it. Come over and help to create a functional set of core apps on nemo with the new UI set that's being cooked and you'll have the fully open stack you like.

And with the quality work of Jolla supporting our UI I think we'll have the best product possible within the "fully open" world.

qwazix: I get the need to be free to experiment without Jolla's tight control over their vision of the UI. But are the set of Qt components dictating the design of the UI itself? Aren't they just building blocks that still could be shared? Or they dictate some stylistic approach which you also don't want to be bound to?

Yes, absolutely. For example qt-components on fremantle look like harmattan, and that is what prompted marxian to create a more native-looking set (http://talk.maemo.org/showthread.php?t=85955)

Qt5 has qtQuick controls which are the building blocks you mention, and they are open. We will base our work on those. (If I'm not mistaken Sailfish Silica are not based on QQC)

Qt Quick controls are documented as Qt Quick components for desktop interfaces. Are they useful for touch interfaces in this case?