Perhaps I'm misinterpreting Alberth here, but I think the point might have been that patch packs don't include tramways...
I suppose the weirdness about tram engines only being available via third-party NewGRF is that this doesn't happen with anything else like, say, electrified rails, which default electric trains are set to run on. But since TTDX didn't have trams (unlike electric trains, which it did have), NewGRF have to come to the rescue for those. Perhaps an alternative would be to provide trams in OpenGFX, but then again OpenGFX+RV already does that.
Adding a Newgrf into the baseset would be an option, but it enlarges the maintenance effort of the OpenTTD program. Most devs prefer hacking C++ above hacking NML or NFO, or they would make NewGRFs instead of program code. It is far more efficient to keep it separate, so anybody can pick it up and improve.
(Ironically, that often means everybody thinks someone else will do it, but that's a different topic.)
A second advantage of keeping trams NewGRF out of OpenTTD, is that you're not stuck with it, like we are with the default set. I think if we would make OpenTTD from scratch today, we would not add any vehicle at all. You download the program, you download your preferred baseset, and you download your preferred vehicles.
Finally, a third advantage is that it keeps a level playing field in the competition of the best trams. Personally, I think this is the most relevant reason. Every author of a tram set should have an equal opportunity to get picked by a user. If we would bundle tram set X with the program by default, we'd disturb that competition.
You see this with the default set again. Although I still enjoy playing with it, the set isn't that good for current OpenTTD. For example, it completely breaks with non-default cargoes. However, since it comes with OpenTTD by default, it is very hard to get new players away from the default set.
I don't know the state of the installer these days since I only use the nightly version, but I'd think perhaps the installer should recommend downloading OGFX+RV to cover trams by default (because there's no guarantee players will even know about NewGRF or otherwise seek them out).
To me, OGFX+ is a set of NewGRFs just like any other NewGRFs. It's specific aim is to enhance the default set, so its name is quite appropriate, but they are not related to OGFX itself.
Automatic download of some set X would tilt the playing field heavily towards that set X. In my view that is unwanted. I am all for a way to make it easier to use NewGRFs, but the first thing you would need there imho is a good web frontend for bananas, so an author can tell about the set, show some pictures, and give highlights or whatever.
Right now, to a new user, a NewGRF/AI/Game-script is nothing more than a random name that means nothing to him/her (eg "BusyBee"), and it's close to impossible to get any solid information about it, unless you want to search forever. You can also not find a good list of stuff that is available. You cannot find what others think about it. You cannot find what combinations of NewGRFs are useful. There are many good sets, but it's not disclosed in an accessible way, so users can browse, compare, download, give it a try-run, comment, etc.