why not run the 64 bit engine? (and why should I?)

Are there any good reasons for not running the 64 bit engine?  (There must be, cause there is a checkbox to enable/disable it...)
 
I assume the 64 bit engine gives more headroom and helps reduce overloads within FX processing?
 
Thanks
 

That's it in a nutshell; to expand a little on what Sander wrote:
 
The standard 32-bit engine provides more than enough headroom for processing and summing without running out of numbers to represent the summed signal; the 64-bit mix engine just provides higher precision to minimize rounding errors as many signals are added together. The main reason not to use it would be that some plugins don't work well with it, and some releases have had bugs (or still do) that only manifest in the 64-bit mix engine (usually causing nasty pops or distortion that is not easily overlooked).
 
Also, it might theoretically add something to the overall processing load, making it necessary to increase ASIO sooner as the project complexity increases, but I haven't ever seen a significant difference in this respect
 
I generally leave it off, and just enable it in the bounce/export dialog for offline rendering. Alternatively you can leave it on all the time and just try to keep in mind when you encounter a problem with playback or mixdown that you should try disabling the 64-bit mix engine as a troubleshooting step.
 
All that said the problems are pretty few and far between, and you may never need to turn it off. 

If a radio station ever calls and says "We were going to play your single, but it's obvious you didn't check the 64-bit box - so we're not interested any more," let me know 

Interesting conversation.. I checked it off based on the more is better theory. I guess I was heavily influenced by Spinal Tap..  I'll check it off and see what happens.. Are there any issues checking it off in the middle of a project?

dlesaux
I checked it off based on the more is better theory. 

Theory refuted: