The guys at Splash Damage are OCD about balancing. They are not going to have one class be ridiculously under or overpowered. And I've seen that video too. You're right. Medics are going to make ridiculous amounts of XP just for reviving people and using buffs and so on. And just because you haven't seen anything on that objective wheel thus far does not mean there will not be.
The guys at SD have been working out how much xp each class gets for class actions vs just killing. They have said they want to make it so even the Soldier (the main "killing" class) is a viable team player, by awarding good amounts of xp for handing out ammo (probably more than you would get for killing guys.)
As for the advantages vs disadvantages of playing Medic - (in general, not just for Brink)
PROS:by far, the most fun class to play
tends to be the class other teammates are most willing to help out. (they acknowledge your importance) Holds more true if you are a
good medic.
the backbone of any team
other than skills needed to complete the core objective(s), Medics provide, overall, the most important skills/abilities for the team.
XP wise, they tend to be the class that accumulates the most, the fastest.
Based on their abilities and their tendency to be the least played class, there is almost never a situation where it is bad/wrong to play one.
CONS:you are a high value target
Other than the Operative, I would say it is the hardest class to master.
typically is a "sidekick" class - A player won the match by arming the charge, but you were the one who kept him alive. He gets all the glory.
typically, they are Incapable of doing core objectives (not in BRINK)
they tend to have the weakest weapon (not in BRINK) and/or have the least ammo of starting ammo (not in BRINK)