You see, with LVM that 'moving other partitions around that are not as full' case -- always rather hair-raising -- disappears. All you have to do is possibly shrink one fs and expand another one.
I've ended up leaving most space in a VG free until I need it, which means I hardly ever have to shrink fses at all (a good thing because most fses cannot be shrunk without being unmounted). This makes expanding an fs that's out of space, no matter whether it's immediately followed by another fs or not, a matter of five minutes' work with no unmounting, no loss of service, and a near-zero chance of data loss. You just *cannot* do that without some LVM-like indirection layer (in btrfs and ZFS's case, inside the filesystem itself).