An old WEG player and GM, Medpacs where one shot use...
We carried over this rule from WEG, along with all our characters and storylines...
Since medpacs use encumbrance, each players usually dont carry more then 2. So they know they can't use them sparingly. They mostly save their medpacs for critical injuries. As GM, I almost always deliver a critical injury when i've got enough advantages for it... Criticals make the fights more fun and keep the characters on the edge of their seet... Since they know I use them fairly often, they know they can't mess around too much.
Looking at the rules, I've found something akward...
You can only make 1 med check after per encounter, but you can make 1 roll per week per critical injury.
In my group, if you suffer 2 crits in one fight, you can only heal one with the med check for the encounter... the other can only be healed one week later. This made crits more threatening.