actually i have been in plenty of wz's that lasted over 90secs without one death, in all 3 wz's in fact, yes it was a good team of good healers, but i still used my reusable medpak, the wz med pak and kolto regen, even my 32 sorc alt doesnt always die on every encounter in every game, and i have needed a med pak heal that was 1-2 mins from refresh... it does happen i assure you.
And you are constantly entering an exiting combat in PvP.
Also, in PvP, your group does not put you in combat if they are in combat. This is obvious, because you can, for example, meditate or stealth during Warzones while fighting is going on elsewhere.
The restriction is "per fight", not "per warzone", or "per flashpoint." PER FIGHT.
There is an icon which appears above your action bar. Its a yellow eye when enemies are near, and its a red "gun" in a circle when you are in combat. When that red gun disappears, you are no longer in combat. The medpack lockout it tied to that red gun (to being in combat). If you are "in combat" and you use a medpack, and you remain "in combat" for 91 seconds, your medpack will be locked out, unable to be used again. However, the SECOND you exit combat, the lockout is removed.
This affects Boss Fights in PvE, not PvP at all.