I have to admit I haven't used TAS for a long while, and the output does look better than when I did. Is it possible to break heat loss down into different elements, eg walls, windows etc? This is something that a degree day model calculates explicitly, so you can see straight away what the big issues are, and say whether the thermal bridges are a problem, or whether there is much scope for an improved glazing spec etc, without having to try a range of options and wait and see what the final result is. My fear is that because full thermal modelling lets all the heat loss paths and thermal mass interact at each time step, it is impossible to disentangle the results in a simple way - but I would be interested to be proved wrong!
The other aspect is just speed - and doing a relatively quick-and-dirty model asap will be better than not doing anything at all as there isn't the budget for full TAS analysis, say when no one wants to spend much money before they've even got planning permission. I don't know how quick you can run through a whole variety of options in TAS, but I know with a spreadsheet you can set up a watch-window on the results you are interested in and get instant feedback to running U-values up and down, or adjusting the window area, etc.