Hello,
Now that MATE has completed the transition to GTK3, is there a reason to maintain forks from the GNOME 2 era rather than use the ones from GNOME 3 which don’t rely too heavily on dependancies?
I’m thinking about File-roller instead of Engrampa, Evince instead of Atril, Gnome-terminal instead of Mate-terminal, Gedit instead of Pluma and such?
Yes (keep the forks)! The GNOME versions follow GNOME 3 design guidelines, so, for example, Gedit doesn’t have a normal menubar but Pluma does.
Isn’t it patched in the Ubunto repos to remove the CSD titlebar?
I think it is, but the patches can only do do much — I believe Gedit still has a Chrome/GNOME-style thee lines menu — and the patching has often kept Ubuntu from having the latest release of Gedit. More importantly, the MATE apps now share the same versions of the underlying libraries, so Gedit and Pluma use the same GtkSourceView and GNOME Terminal and MATE Terminal use the same VTE library. This means they automatically share a lot of code related to their function of e.g. text editing despite the app interface being different.
But don’t me wrong, while I think these core apps should match MATE’s design, I love that Ubuntu MATE has never been obsessed with “purity”: the default video player has been VLC (Qt!) and it ships with a few GNOME utility apps with header bars that don’t make sense to fork (GNOME Disks and previously Dconf Editor).
Don’t get me wrong either 
I don’t have a problem with the MATE apps, nor do I think that the GNOME ones are better, it was more of a “shower thought”: when MATE was started because people didn’t like the GNOME 3 interface, it had to fork everything because it was still in GTK2. But now that it’s in full GTK3, I was thinking that reducing the number of forks might help the developers concentrate more on core parts of the desktop rather than putting time in maintening what are basically the same apps as existing ones from GNOME.
MATE should be a complete DE. Keep Gnome apps as far away as possible. It would make more sense (from a UI perspective) to use X-apps from Linux Mint, but don’t do that.
MATE already has the best apps, IMHO. 
I personally like MATE’s apps as they still have the GNOME 2 look and feel, even if they’re GTK3 now. Long live the traditional desktop!
The other problem with bringing in GNOME apps by default is that the dependencies would grow and bloat the distro.
I agree with @lah7. And @mrtribute. MATE is a full desktop experience with uniform look and feel. Ubuntu MATE may have some Qt apps (ie. VLC, as mentioned before) but they blend in well and don’t make the whole feel like patchwork.
Let’s not forget why MATE came to be in the first place. GNOME apps may “fit” MATE desktop and core system right now, but who can guarantee that is the case a year or two years from now?
By maintaining its own core apps MATE is in command of its own destiny.
I can’t think of a single MATE app that i don’t like better than the corresponding GNOME app.
That said, i use nedit or gedit instead of pluma, and have had no problems with collisions making things fail.
Just for the record, we won’t be replacing MATE applications with those from GNOME.
We, may, include some GNOME applications in the Boutique that add desirable functionality not available in MATE. We already use LightDM GTK Settings from the XFCE guys and I may expand that to MenuLibre in 17.10 because it is simply a better menu editor than Mozo, despite having headerbars. That said, the headerbars have much improved styling in Ubuntu MATE 17.04 onwards and far less objectionable IMO.
The MATE applications will remain and despite their traditional user interface are every still closely tracking GNOME3 for backend functionality. We regularly sync MATE applications with their GNOME counterparts, yet we preserve features and the traditional UI. So don’t assume that GNOME3 apps are some how newer, they are not. The backends are almost identical 
Great replies here that pretty much echo my sentiments. I also believe that a distro having a robust set of it’s own core apps helps with branding and ties in to the point regarding MATE being a full desktop experience, a detail that even a newer user is likely to pick up on. While there isn’t anything inherently wrong with a distro using tools from elsewhere, it can make it feel a bit hodge podge and perhaps lacking it’s own identity. Even if functionality is identical to it’s Gnome counterpart, I always enjoy seeing and using the MATE version. Though it may add a little overhead to maintain I feel the effort is well worth it for the previous reasons and it also should help to isolate MATE from the “NIH” syndrome that can ocassionally plague a DE.
I think it’s good to separate MATE from any weird curveballs the GNOME devs could throw our way.
That being said, if I had to pick one MATE app that could benefit the most from grabbing a few more features from the competition (though more from Nemo than Nautilus), that would be Caja.
A configurable toolbar would be soooo awesome, for example.