See, now that's not the reason at all.
Yes it was the original game-mode, but surely it's also the best? The reason you'll see TE winnings 'hands-down' is because it's simply the most popular choice for anyone wanting more than 5 minutes of continuous tactical play.
It's all about attacking, defending, and giving two-fingers to the other team.
Countdown is good fun, but with one life, one death, and a few minutes of watching your team run around it can get a little tedious. That's not to say it isn't exciting or enjoyable, it is.
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Countdown to me is more like "countdown to a nose-picking." Boredom personified. Especially on some maps where it's often, take a few steps towards objective and be instantly killed by dirtbag exploiting a map weakness.
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Firefight is, in my opinion, a poor man's Team Death Match. Why a poor man's TDM? Well you get 5 minutes in a round to kill or be killed, over and over until the clock stops. It would be OK but the fact you spawn in random places, often surrounded by enemy, simply makes it less fun than it should be.
What was so wrong with dedicated spawns?