That post brilliantly sums up the issue facing the majority of most town/city centres. Retailers have moved to out of town shopping like Trostre and MacArthur Glenn as rents are cheaper, units are bigger, parking on the doorstep, etc. Every retailer moving out of centres means there's less and less there to attract footfall, so more and more make the move. It's a vicious circle. So while you are correct in saying there's 'nothing there' in a retail sense, there's lots of other stuff to do in the city centre, which is what will likely become the norm in the future.
On another note, I had to go to Cardiff for work yesterday. I normally only go to Cardiff maybe once or twice a year, it's always on a weekend and I drive up. Yesterday I took the train and arrived around 8:30. I previously said in this thread that you can't compare Cardiff city centre to Swansea's and yesterday underlined that for me. Getting off the train and seeing the BBC offices, the bus terminal, the UK Government building (not been up by train since all that was finished) and the throngs of people on their way to work, it was another world to Swansea city centre. It felt more like walking out of the Underground at a London station.