I find UNC paths far better to work with. If I have to reinstall the OS on my HTPC or laptop, it's far easier to just restore the MC library and not having to worry about remapping all my network drives and getting them all right.
By the way, I agree on this too. There is, however,
one case where using a mapped network drive works nicely with MC, and is why I use M: on my home server instead of pointing it to UNC paths.
I have my main RAID (drive M: on the server) set to back up automatically to
one of these things. So, that portable drive generally has a backup copy of my entire main Media Center media collection (less mostly kids shows that I don't care about to save space).
I take that drive with me when I travel, and plug it into my laptop and mount it as drive M (so it is both a "backup" and a "portable media drive"). But since it is still drive M, and my main Library has all of the files imported on the server's drive M, then I can just use my regular Library and access the files without any modifications to the Library or re-importing or anything.
My backup system makes a copy of the Library itself on the backup drive, so I can use that. But, even better, because of the way the
Play local files feature of MC works, I can also still access my "real" Library running on my server, but since all (or most) of the audio and video is on the backup disk, and accessible locally, it doesn't need to stream everything to my laptop, but can just play them.
If new stuff has been imported since the last backup, I can still get that too (those will stream). But most stuff is there, and they work without all the transcoding and added network latency and all of that (pretty essential for video, if you're tethered to a cellular data plan or on cruddy hotel wifi or something).
It works awesome.
But, for all of my other Libraries that I use for work or whatever, I always import everything via UNC paths for the exact reason you stated. Way easier for accessing from multiple machines, without doing a bunch of drive mapping nonsense (which you have to fix on perhaps multiple machines if you ever change anything).
In fact, I sometimes
want to switch my main Home Library over to UNC paths, but that "portable drive" use-case is compelling enough that I've always ended up sticking with what I have. I combat the inconvenience by ensuring that I use ONLY that M drive, and if I need more space, I expand the size of that volume (rather than using multiple volumes).