You cannot poach an entire team, or even half a team, easily. The point of the acquihire for me is either to acquire specific members/leaders of a team (that wouldn't be interested by just being poached anyway) or an entire team/specific knowledge.

Sometimes a startup reach its peak and feels its the right moment to make a change. An acquihire can also lead to that.

You can search about the Sparrow acquisition (a lot has been written here about it). Another example that comes to mind for me is Google acquiring Wildfire and the team reaction when they learnt it : http://www.youtube.com/watch?v=4ARPB7b0kIg