you are using references for already measured energies and orbital systems. For example, if you take the measured energy change for, say, but-1-ene and the estimated heat of formation difference for a C-H vs. C-C and hydrogen, you will get a good estimate for {alpha,omega} octadiene.

If you think you can do the same thing for ethene -> butadiene, you'll find yourself to be horribly wrong, because of extended conjugation networks.

So while for some cases it works, it is not always simple to go from known empiricial results to more complex structures using tables and addition and subraction. And in the case of the molecules I care about, there is pretty good reason to believe that the simple linear methods will fail.