To me this seems like a somewhat volatile topic, so I'll keep my personal opinions out of it and stick to just fact correcting.
Simple fact of the matter is that every week Evo has something shiny and new. For example, 6 of the old models just got replaced with lego models, they have new effects, new sounds and all other kinds of jazz. When alpha/beta builds get released, there will be a weekly "patching" cycle.
Yes Evo is very polished, and is in itself a very full rts game even though it has a single faction (However that single faction has no less than 4 full techs, each with it's own particular flavor) (Reasoning being, I would rather have a single awesome faction, than 2 mediocre factions). The missing elements are only in the form of the gui, however, evo uses metal and energy as well, so using the default redui, lolui, iceui, setups works fine, it's just that we would like to have our own(tm

) gui. There are no bugs of which I am aware. Anyone that knows me and my habits can attest that I do not leave broken code in a repository.
Evo also has a single player menu when launched via spring.exe (thanks to z). The menu isn't quite done being sorted, but it's fully functional and nice looking).
***end of fact fixing***
Ok back to the topic at hand. I don't think that KP would be a good choice for inclusion with the installer. The reason is mainly because of the graphics and effects. Yes, KP achieves it's purpose, and it does it really really well, but that purpose is to
look retro. In my mind, engines that release games with them always have that game as a techdemo. I don't think any or the projects in spring approach techdemo status, and if they did I would attest that CA, Evo, and Gundam are the closest you would get atm (I'm speaking more of the gundam dev builds than 1.21 - most of you guys don't see the incredible bout of awesome that smoth has been working on lately).
Moreover, I think even releasing an installer for the engine is a MASSIVE mistake. Allow us gamedevs to stand on our own two feet, and stop circumventing us by releasing an engine installer. Using shared spring folders it's already possible to have multiple games spanning the same content. Let US do the heavy lifting here!
Also, see this thread:
http://springrts.com/phpbb/viewtopic.php?f=1&t=23467