I stand corrected - service careers do indeed have mods. I'm not aware of any others.
Insofar as "close calls," that's a good way to run it, especially since Prior History is supposed to make the character's background more "real" to the player and the rest of the group.
My 14-year-old son plays in my D&D campaign and is not the strongest role-player in the world. I love him; I'm very happy to be playing D&D with him; but he's only interested in the hack 'n slash, pretty much. Politics? Boring. Intrigue? Boring. Three-headed dragon of death? Booyah!
That being said, I sat down with him to try making a few characters in T20 as a learning exercise for me-as-GM, and we rolled up a belter, an academic-from-hell (man had 10 googol skills), a merchant, and a noble. My son really liked the noble, who had an extremely colorful and extremely long (9 terms - two mandatory reenlistments....go figure) career.
And he said words every GM who reads this want to hear their players say: "You know, you can make up a whole background story from these Prior History things and create family and friends and enemies and who got killed and...."
It's one of the things I **LOVED** about Traveller, and it remains so.