My standby has always been my ISD2, mostly for the defensive retrofit and the longer range guns. But the ISD1 is still a better general purpose ISD than the Kuat or the Cymoon. Like as been mentioned before it is the cheapest ISD and getting it a Defensive retrofit cost the same amount of points as upgrading to one of the new ISDs. So just cost wise it is at least no more expensive or the still the cheapest, depending on what you want to do with it.
I have actually found that and ISD1's black dice can serve a defensive role. If I want a carrier or a flag ship to stay a little back from the battle, those three black dice discourage, MC30's, Raiders, or Demolishers from choosing to attack my ISD1.