They're certainly not agile, nor are they perfect. No question about that. But they do have a long history of ignoring Wall Street while working on building long term profitability. The Xbox360 and the Zune are good recent examples. Though the Zune has yet to materialize as a major profit source, and honestly may never, they're definitely going against the wishes of investors to try to become more of a player in the consumer space.
My personal feelings aside as to whether or not MS/Yahoo is a good idea, Cuban is right in that it will allow Yahoo to focus on the long term in a way that remaining independent (if that's even an option, which it probably isn't) will not.
There's definitely some logic in that, but my gut feeling is that all the 'hullabaloo' surrounding the merger would supersede that effect. Hirings, firings, integrations, strategies, turf wars, technology conflicts, etc... will not be an easy thing to keep under control and out of everyone's way in order to let them concentrate clearly on the future. If they manage to, then what you're saying will come into effect.
My personal feelings aside as to whether or not MS/Yahoo is a good idea, Cuban is right in that it will allow Yahoo to focus on the long term in a way that remaining independent (if that's even an option, which it probably isn't) will not.