You're better off farming kills or support points. Kills are 100 XP each, plus a potential +10 headshot bonus, plus occasional bonuses for ending an enemy kill streak, plus bonuses for you getting a kill streak. Get ~4 kills in a life, and with the bonuses, you've just earned a cert. Support points are also nice - reviving a soldier grants 75 XP, and the soldier comes back at low health, awarding another 40 XP if you heal them back to full. Alternately, an engineer can get ~150 XP from repairing a destroyed generator or turret, making amp stations an awesome source of loot.
Unfortunately, this makes sense, and it kinda bothers me. When you put it that way, and I think you're right, it seems like they're encouraging you to wade into the meat grinder over and over, grabbing a couple kills every respawn. I feel like I'm helping my faction out a lot more when I go capture 4-5 sparsely defended areas in the same time that it takes for one massive clusterfuck to grind itself out. I have more fun doing it, too... but I only get like 15-20 certs per hour that way.
Or, ya know, admit that you're having fun with the game, decide that it's worth $20, and buy the weapons you really want. Sure, it's nice that everything can, in theory, be available for free... but I'm quite willing to drop $20 on a game that could easily have been a AAA release at $50-60. Think of the F2P mechanic as a really awesome demo (something that you don't see a lot of games releasing any more), and suddenly the pricing of items starts to make sense.
I wouldn't actually consider this a AAA title. It can be fun, but at best I would consider it unfinished for a AAA. The engine needs optimization. I see player positioning bugs constantly. There's more reports of cheating every day.
Even if it were a completely solid AAA title, I still wouldn't pay that much. I buy games almost exclusively during Steam sales anymore. During the Fall sale, I picked up 12 games for about $30, and a couple of them were AAA titles. By comparison, how can I justify spending $10 on a single item in a single game? For 1/3 of the price, I'd consider it. You could argue that they have to jack up the price so that paying customers make up for all the freebie players, but they'd make much much larger volume of sales at impulse buy prices, which I consider to be no more than $3. I was actually planning to buy a some in-game cash the other day, thinking I'd drop $5-10 just once and buy a few items. Then I saw how expensive everything is and just couldn't justify it. That's when I logged off and made my first post on the subject in this thread.