If you have the right job for the tool... these tradeoffs should make a lot sense.
But it seems like the claim made on http://www.mikealrogers.com/2010/07/mongodb-performance-dura... that with an in-memory version of Postgres "once you turn off the log and all the durability it’s neck and neck with MongoDB write performance" ... might not be so far off the mark.
I still like MongoDB though. During dev it's nice to not have to worry so much about schema, run ALTER, etc.
It's not a trade-off if there's no upside.