Getting off topic, but
which for some reason cannot be installed concurrently on a system
Don't know about Windows, but on my Linux box
$ ls -ltrd /usr/local/stow/ard*
drwxrwxr-x 8 boffin boffin 4096 May 18 2013 /usr/local/stow/arduino-1.0.5
drwxrwxr-x 8 boffin boffin 4096 Sep 17 2014 /usr/local/stow/arduino-1.0.6
drwxr-xr-x 10 boffin boffin 4096 Aug 28 2015 /usr/local/stow/arduino-1.6.5-r5
drwxr-xr-x 11 boffin boffin 4096 Nov 4 2015 /usr/local/stow/arduino-1.6.6
drwxr-xr-x 11 boffin boffin 4096 Dec 18 2015 /usr/local/stow/arduino-1.6.7
drwxr-xr-x 11 boffin boffin 4096 May 10 2016 /usr/local/stow/arduino-1.6.9
drwxr-xr-x 10 boffin boffin 4096 Dec 13 00:02 /usr/local/stow/arduino-1.6.13
though to be fair since they all use the same preferences directory, it's a bit iffy to go backwards (usually if I need to check something in an older version I'll move my prefs out of the way first).
some of the later versions break large amounts of older code
Well you can't maintain 100% backwards compatibility all the time, things have to change. But that said the Arduino IDE release QC process is.. well they don't have one, some releases of the IDE (eg 1.6.10) caused all sorts of problems.
I wish they would have a proper release cycle, with beta releases, so that the people who are capable of debugging problems can do so, before it got to a stable release for the masses.
The whole development of the IDE is a bit... weird really, for a high profile project, I think it boils down to a few "gate keeping" egos in there doing things their way, a lot of good ideas get pooh-poohed, they do IMHO crazy-ass things like writing it's build system (arduino-builder) in what really is a pretty obscure language (Go), the afore-mentioned lackadaisical release cycles...