By "sensible" you mean "disastrous." Owning Sony Pictures has been a consistent money loser, and the studio IP concerns at Sony Music and Pictures hamstrung their electronics arm from innovating in the face of the iPod threat.
In Sony's defense, the idea of a modern electronics company owning actual content was an interesting idea, especially as they introduced new media formats. It's not far removed from what Amazon is attempting in publishing and what turned out to be a very successful model for the game industry. Unfortunately for Sony, it turns out that the economics just don't work for music and movies.
It's more than the economics of owning a studio - Sony's ownership of their studios actively harms their other, actually profitable, product categories.
Look at the PSN's movie rental store - perpetually hamstrung by limited selection. Why would competing studios want to get into bed with one of their biggest competitors on their closed service?
Similarly, it creates internal conflicts of interest. Sony's electronics are constantly crippled when it comes to content, probably at the pressure of owning their own content themselves. For the longest time it was impossible to get any movies on a memory stick, forcing PSP users to buy them on expensive and battery-murdering discs instead. It took years to fix that mistake, but of course, by the time they did, public interest in mobile movies had basically all but disappeared.
It was a very 'sensible' decision if you are an MBA! If Sony had been a little more flexible and far sighted it could have worked. But as you say, it resulted in the DAT tape and minidisk being restricted to protect the studio.
It's the same sensible that means Apple shouldn't have started iTunes (we aren't a record company) and Amazon shouldn't have done Kindle (we aren't a computer company) !