For starters, job creation and economic investment. I know the area near the SSC and almost everyone there will tell you when it was under construction the entire area was projected to undergo a massive economic boom. It started to do it too until the project was cancelled and then things stagnated in the area pretty badly. It has only been in the past 5 or so years that there has been significant growth for that area because everything changed direction.
Dallas now has several major economic sectors, but scientific research is way behind for it. If the SSC had actually been completed, that area would probably be one of the largest scientific hubs in the world by now and we likely would have found the Higgs (and other things) a lot earlier. This in turn would have generated a much greater demand to come to the US for STEM type education and research and likely would have helped our post secondary and general secondary education system flourish as opposed to what is happening now with them struggling to keep up with rising costs (especially in Texas, where the secondary education is abysmal in most areas). If even a small ROI had been shown that probably would have helped the scientific community in the US in general with some of these larger projects and we would likely being growing that sector much better than now (funny, that is now what half the country has talked we NEED to do in order to continue competing on the global market...).
Other industries would have also been able to grow out of the construction and maintenance efforts and Dallas would probably have seen some amazing growth from that alone. Not to mention the indirect benefits of growing the field causing other improvements in the long run. Big Science is like IT, very necessary and can have tremendous benefits, but since you can't easily quantify all those benefits on paper people will shoot it down (kind of ironic considering that is the entire point of most hard science fields, quantification and making it easier to understand).