Its still not going to destabilize the OS. iOS applications keep their data in the solid state RAM Disk just like every other application.
Though a dictionary with a million entries will be slower to access than one with 100k entries, and you will use up more of your "disk" than I will, it will not destabilize the OS.
Why keep a dictionary closed? The same reason you can't organize photos in the Photo App on the iPad. It has not been seen as a priority for Apple developers yet.
It's just that simple.
You can expect dictionary editing and maintenance tools in a future release. There are lots of dictionary enhancement requests in the request tracking system at Apple for iOS. They will get to it when it makes sense for them from a business standpoint - and I suspect you will have import options at that point in time as well if you want to slurp in a dict file filled with technical references, names, etc.
-t