There should be incentives of some sort, and I think it would be hard for anyone to give a well-reasoned argument why there shouldn't be.
1. There's only so much that needs to be said. I often pass on writing an assassination because I have nothing new to offer. If everyone was writing an assassination on every team, there would be so much overlap in points that actually finding new and salient points would be like finding a needle in a hay stack.
2. If everyone writes an assassination, then it isn't just a time commitment to write, but also to read. No one likes knowing their 30 minutes work was a waste of time and many GMs will scan or not read at all, which means that good points will be missed and not properly discussed because they are missed.
3. The more assassinations, the harder it is to respond to each one. Lets not forget that the dedication of time is not just writing an assassination, it is responding too. And just like writing an assassination, responding takes more time than merely the typing time, it requires that the GM take the time to properly think out their replies and in many instances research their replies.
4. GMs being forced to pump out assassinations is not fun, this whole process is fun. It's a game, not a job, we should never lose sight of that fact.
5. Everyone has a different personality. Some may get a lot more from simply reading an absorbing assassinations, and that is a form of participation in and of itself. A system of rewarding people for writing assassinations rewards those who's personality has them derive pleasure from writing assassinations, while in the long run discouraging GMs who find it tedious from participating in the first place.
6. If the assassinations thread is filled with everyone's opinions in full detail, it will provide a road map to how voting will play out, removing the suspense, surprise and mystery from the playoff process.
7. There is already so much to the process that rewards GMs who enjoy long drawn out debates and research. The system greatly rewards those who have the time and desire to dedicate their time and mental resources to the ATD. Given how these long drawn out debates drain the momentum out of the ATD, it is clear that we should not be trying to increase the prominence and reward for debate. This plays into the personality type argument, the more systems like this we have in place, the more we encourage one group to gain joy from the ATD and the more we discourage all other groups from enjoying the process.
So, in summary, this is a game, this is supposed to be fun. We should be setting in place rules that make the ATD fun for as many different people as possible, not as few as possible.