Pretty much every binary in /bin in Debian (the OS I have at hand to check) links to a library of some sort. Therefore, if my /lib is hosed, I can't use any of the tools in /bin (I've had this happen, and I couldn't even use "ls". This would not have occured if those binaries were statically linked (I believe OpenBSD does that for /bin and /sbin).
Agreed. If distros wanted to try and champion the cause for a statically linked subset of critical coreutils, they could still do it in the merged tree. The only thing the separation would facilitate would be separate mount points or physical devices.
There are examples for both approaches. OpenBSD keeps /bin minimal and statically-linked, and it works. Solaris merged /bin and /usr/bin a long time ago, and -- unsurprisingly -- that works as well.
Frankly, as long as it doesn't bork my Debian machine when I move from Jessie to whatever Debian 9 is going to be called, they can merge all they want. At this point, seeing how many Linux systems are doing the merger, it's probably a good idea to do it, too. Packages whose developers give a flying fsck about portability (fewer and fewer nowadays because lockdown and unportability were bad only when Microsoft were doing it) already deal with both usr-merged and usr-unmerged systems (hardly rocket science in most cases), and the maintainers of packages whose developers don't give a flying fsck about portability and only develop for Linux can hope that four years from now, everyone will have merged, especially now that Debian is doing it too.
I think the fact that I can't even remember the last time I cared how things in /bin are linked is good proof of how important this "tradition" is, at least for me...
The merge does involve some loss of flexibility (for instance, "traditionally", you could use the programs in /bin to recover a failing system), but there are fewer and fewer users relying on it. It also involves departing from the FHS, but Debian already does that (e.g. they use /lib/<GNU triplet> rather than /lib{32,64}). I haven't heard any super convincing arguments from either side; normally, I'd go by the "not broken -- nothing to fix" route, but it's also hard to poke holes in the "this additional complexity isn't needed anymore" line of reasoning. Plus, like most arguments that involve tradition, it's pretty hard to tell robustness from cruft.
Whether this change can be done cleanly and without breaking users' systems is an entirely different story and past experience has shown that moderation is a far better friend than optimism. It's particularly difficult to migrate existing installation to this scheme (e.g. Arch failed quite badly, as I remember from those particular two afternoons, thanks a lot Arch!, and it didn't go more smoothly for others, either), but Debian can benefit from the lessons learned by more up-to-da^H^H^H^H fast-moving distributions and do things better.