The BS wasn't the only thing being upgraded. The cruiser was as well. Cruisers would still be very good. And as he said, 2-3 would be able to take down a battleship.
Plus, if someone is ranting around in a BS, another BS owner is likely to stop them.
Also, moving your one BS somewhere away from your faction's system would severely lower the defensibility of your fleet. Leaving the system open to being attacked by someone else's BS. A BS can kill anything else in this situation, yes. But you still have the option of retreating if your a lone cruiser against a BS. Heck, right now a lone cruiser can't beat a BS anyway.