While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of BlackBerry (NYSE:BB) slipped 1% this morning after Macquarie downgraded the embattled smartphone maker from neutral to underperform.

So what: Along with the downgrade, analyst Kevin Smithen lowered his price target to $5.50 (from $7), representing about 15% worth of upside to yesterday's close. While contrarian traders might be attracted to the stock's big decline in recent months, Macquarie believes that BlackBerry's appreciation prospects remain limited given the seemingly slim chance of a breakup.

Now what: Macquarie lowered its subscriber forecast for BlackBerry's fourth-quarter 2015 from 53.7 million to 46.8 million which, in the firm's view, should lead to a services/software revenue decline of about $50 million. "Given the recent $1bln convertible, an analysis of recent 13-D filings from Fairfax [Financial] and comments from new Executive Chairman John Chen, a break-up no longer appears as a likely outcome, though still possible," Macquarie noted. "Meanwhile, our checks indicate a pick-up in the pace of services erosion at US enterprise customers, which will offset much of the 50% opex reduction, in our view." Without the real prospect of a break-up, it's tough to disagree with Macquarie's downgrade of BlackBerry as a going concern.