/mnt is for whatever you want it to be
Which doesn't mean that you have to be standards-compliant....
And the difference between my statement and yours is.................................?
Not very great.
Well, Gregg asked for a "standard" place for his data partition. Then he confessed to being "obsessive about the 'accepted practices' thing". Now, a directory under
/mnt is decidedly non-standard as a mountpoint for a permanent partition.
Which doesn't mean that your setup doesn't work for you or wouldn't work for Gregg if he copied it.
The same is true about mine by the way: I mount my main data partition (labelled "Data") as
/home/<myname>/Data, and most of the data folders in my home directory are just symlinks to directories under ~/Data. So even if my data partition is mounted at a mountpoint under my home directory its contents are kept separate from the configuration files in that same directory.
One benefit is that I can reinstall the system without having to recreate the mountpoints under
/mnt. And even if I reformat
/home I won't lose the data on my data partition, although then I would have to recreate a mountpoint and a few symbolic links afterwards. If your
/mnt is on a partition of its own, or if its subdirectories are on partitions of their own, the difference between your system and mine is really marginal. (Except that mine is more standards-compliant.)
/mnt is for whatever you want it to be
The same could be said about most standard directories. (At least so long as one doesn't try to create permanent mountpoints under
/proc or
/dev or
/sys or
/tmp.) And I'll gladly deviate from any standard if I see any benefit from doing so. But if I don't, I won't.