I've been thinking about this for a bit.

As an Army vetran I have no problem with anyone wearing the Army tartan so long as they do not try to represent something they are not.

Honoring a relative or a friend is a fine application.
Wearing a "service" tartan because it looks great is good as well, but with a caveat.
If you don a "service" tartan know that there are those around you that will know what it is and what it represents. Dishonor that and things could get ::cough:: rude rather quickly.