Your example of leaving 500GB free space for use by either video recordings or experimental software is easily handled with raw partitions and GParted:
1. Create the video and software FSs, say 200GB each, on partitions
2. Leave 500GB unpartitioned free space
3. Decide to expand (say) the video FS, which is before the software FS
4. Use Gparted to move the software FS up by (say) 100GB, and expand the software FS by the same amount.
5. Wait while Gparted does all operations
I haven't included unmount/mount because in some cases using a live CD for Gparted is easier.
My point is that many common FS resizing operations are easily done by Gparted - it will take longer in the 'commit' stage when it does all the work than with LVM (due to moving the data blocks around), but you can leave your system running.
For the typical laptop/desktop case, Gparted + raw partitions are much easier than LVM commands + LVM. For servers where uptime is important, LVM is much better, but it requires some care to get all the commands right.