> As far as I'm concerned, per-user application installs really don't make much sense nowdays. Almost all systems are single-user, so there is only the one user to install the app for. The few cases that remain where there are multiple users on a single system image are all cases where it's probably a bad thing to have users installing software (especially given how easy it is to have separate system images with either virtual machines ot containers to isolate the users when you do want them to share hardware)
Even on a single-user system, there are many "users" like apache, ftp, sshd, dbus, etc. And these programs often have security as a high priority, and would not like libraries being switched out underneath them.
So it's nice to be able to install applications and libraries in my own home directory, even though I'm the only human user of the system, just to avoid upsetting the rest of the system.