MAPI is just one part of what will be available in the next release. There are a TON of things scheduled for the next release: All of the sharing improvements, the re-written and greatly improved protocols like EAS, EWS and IMAP, all of the webmail performance improvements, back end file system changes and system performance upgrades, the synchronization improvements to non-MAPI protocols, etc. will all be available once the "MAPI" version of SmarterMail is available. So yes, if you DO test the BETA, there are other things you can review other than MAPI.
As for a public release between now and when the BETA ends, we did just put out a public release on the 1st. If there are significant bugs that are impacting a large number of customers, we'll put out another one prior to the MAPI release. However, we are very close to having things wrapped up with MAPI. (And everything else in the release.) So the next version is essentially feature-complete at this point, it's just a matter of cleaning up the bits-and-pieces and continuing to test it as much as possible.