This build is realy usefull if you have no invisibles in your team.
The enemy wont expect you building shadowblade and having 12 seconds invisibility is in some ways even better then 12 seconds invurnability.
The trick is that activating shadowwalk will not deactivate your ghosts, so this gives you a lot of time in teamfights to just eat away your opponents.
also it is realy usefull for escaping.