The thing is that ursa is not an initiator. Yeah, earthshock is a pretty good slow, but it isn't as freaking good as thunder clap or ECHO SLAM. And while he might build beefy, he hasn't got a sick armor like other agi carries due to it's average agi gain. That's why I consider blink dagger garbage on him. Yeah, you catch people, but they keep running. And no stats for 2150g is just not worth it. And we should also mention that the mana cost of blink drains his mana pool pretty quick. Its as big as the cost of overpower.
Force staff is only 750 g more than atos, but 1000g of that go only on the recipe. While it has more potential uses than atos, it gives you no damage, half the intelligence, less chasing ability and just about the same survivability. Also, atos slow is on a 16 sec cd, 4 less than force staff.
Also, you can initiate on one guy pretty well with atos. That thing has an ENORMOUS range.
By the way, MoM lifesteal doesn't work with fury swipes, they are both orb effects.
My build is usually tranquils, vlad+brown boots for early rosh, atos, phase boots and then whatever is needed (usually bkb or basher, but the last game I had to go with linkens -> bkb as a personal fuck you to the enemy bane)