I don't think Cisco will ever get away from allowing access to the command line.  From what I understand, the SDM was developed only because competitors have a GUI front-end.  Plus, having a GUI front-end opens up the market to people who otherwise would have chosen something other than Cisco.

Personally, I feel the GUI front-ends that Cisco has (other than the one for their IPS) does a poor job when submitting the commands to the device.  The naming convention that the SDM uses is terrible.  I like to give access-lists meaningful names, not generic names like the SDM does.

As for the exam, I would recommend having some familiarity with it especially since what you are reading is mentioning it.  You don't want to fail the exam only because you didn't know where to go in the SDM.  Just my thoughts.

Good luck!