Obviously we're in the minority here, and for the life of me I can't understand it. I'm in 4 dynasty leagues, three of them have deadlines and the fourth doesn't. No one can ever give a real reason for it, other than "this is the way it's always been."
If I'm in a league with no trade deadline, and my likely playoff opponent suddenly gets a significant injury at a position, there's nothing preventing me from contacting my opponent's likely trading partners and trying to "steal" his likely options.
And for the people arguing that the values actually decrease, I think you're crazy. I guess you've never had a key player get injured during the playoffs - lucky you. Let's just give a hypothetical example based on SSOG's top QB example.
For the purpose of the example, Team A has Tom Brady, but because of a rash of injuries isn't going to make the playoffs. Team B has Aaron Rodgers, and is cruising, easily locking up the #1 seed and a week 14 bye. Because of who their starters are, they both have average backups (since they are only bye-week fill-ins).
During Team B's week 14 bye, disaster happens. Aaron Rodgers goes down and is done for the rest of the season. Since there is no trade deadline, Team B contacts Team A and offers Aaron Rodgers for Tom Brady, and Team A accepts the offer. Team A certainly improves for the future, as Rodgers is ranked higher and several years younger. Team B makes the move, because they want to take the shot while they can.
How has this trade "thrown off the competitive balance of the league"? It seems to me that the injury threw off the balance, not the trade. And it seems that in the same situation, a trade deadline is what actually throws off the balance, because it would prevent a team from having a realistic chance to overcome the injury.
And to those that say you have to plan for that, sorry, but you can't plan for injuries. They can happen to anyone at any time. For those in favor of trade deadlines, I hope that something similar to the above scenario happens to YOUR best player, then see how you feel about trade deadlines then.