There are many ways to approach the datum structure and the best is always the one that is the most practical to your system of manufacture and evaluation. Clearly, blindly setting the datum structure to zero as shown on the print has caused many companies to go broke, however, it is the starting point. Iterative alignments have helped zero the structure, but the datums being zeroed to a higher level is insignificant IF A MAJORITY OF YOUR PART FEATURES are not manufactured to these same datums. A more practical approach is to get a 'shotgun' look at your major part surfaces to see how your datum features relate to the part as a whole. THIS SHOULD ALWAYS BE DONE BEFORE ZEROING DATUMS AND INSPECTING THE PART. If not, the datums can be zero and the entire part surfaces off, causing rework to the entire part cavity when a little 'dusting' off one of the datum features may suffice. Some of the new math modeling packages allow 'weighting' the surfaces and are drifting away from the traditional 3-2-1 alignments. It really depends upon the process, part, and maturity of the check system.