This is the proverbial "Easier Said Than Done." The BB user base has to be pretty much down to its original markets: large business and governments. The large businesses represent an opportunity, but then they'll probably migrate anyway over the next few years without Microsoft having to purchase BB.
The government base will be a huge black hole for whoever buys BB. While the end users of the devices will want to change, the secure messaging infrastructure of BB will make this a huge effort that's disproportionate to the return. Unless Microsoft, or anyone for that matter, has some magic pixie dust to enable this change in services, BB going belly up is going to be a huge(!) headache in the government services area that I'm sure no one really wants to take on.
A few interesting takes by the OP and dkediger and I tend to agree. However I will go further and state that believe that a MS buy of BlackBerry, either as a whole or in-part may be a match made in heaven for MS for a few reasons;
First, as stated previously, BB strength is the enterprise and enterprise services. Even though MS does have a reasonable foot print in the enterprise via desk top SW and MS Exchange, BlackBerry BES is still coveted by IT departments for it's security and manageability features. BES runs in front of a good number of Exchange installations to this day and is growing. This would be a good way for MS yet further consolidate their foot print in this sector and add a revenue stream.
Second, BB long time and loyal following in the Smartphone market space has come from users whom covet "GSD" (Getting s*** 'stuff' done). MS WP8, while quite elegant, has missed this mark to a degree (as did BB OS10 at launch). We all understand the the frothing consumer masses that covet social media connectivity and audio / video playback drive the volume Smartphone market now and any company bent on being a real contender needs to service this market trend, it should not come at the cost of basic "GSD". It should be in addition to. Imagine for a minute, some time in the future, a best of both worlds blending of WP8 and BB OS features. WP8 (probably WP9), could gain rock solid messaging, enhanced security, messaging hub concept, user configurable alarms, alerts and other niceties just to name a few.
Third, in my opinion, MS could benefit the most from a BB buy-out by gaining BB QNX. Simply, the QNX RTOS is bullet proof. Variants of QNX already run in mission critical application from military to nuclear control to automotive telematics (QNX is in 65% of auto world wide already) to the kernal of BB OS10. This could give MS a rock solid core for WP, XBOX, Set top box TV, and the spring board for taking MS Sync Auto to the next level.
Last, QNX BB10 already has provision to integrate an Android run time. BB10 user are able to side load Android apps to run on BB10 devices. While not as optimal as a native app., imagine again the ability to source and run "must have" apps on your WP8/9 device to hold you over until the WP ecosystem catches up.........
Oh, if wishes where fishes.........