First, legacy Mating Conditions in existing files will be maintained as far into the future as we can foresee, however, as of NX 7.5 you will no longer be able to edit or create new Mating Conditions.
Second, Assembly Constraints are based on newer architectural principles which have fewer 'constraints' (if you'll excuse the pun) such as with mating conditions you can only have ONE fixed component and you have no control over which one it is (it's always the first component placed) and it can't be edited. Also Mating Conditions are hierarchical meaning that the order in which they are created is critical whereas Assembly Constraints are not and therefore it's easier to make changes later on. It's like a sketch where you can add and remove dimensions and constraints anytime you wish without needing to reorder, unlike features where order is critical. This also means that with Assembly Constraints it's much harder to get into situations where you get circular references, something which is quite common with much more common and frustrating with Mating Conditions (and for that matter with modeling features).
It also allowed us to improve and make it easier to apply things like variable positioning as well as gave us the opportunity to implement some new concepts which would complicated Mating Conditions even more.
This is just an example of where advances in mathematics and software architecture has allowed us to make a significant enhancement to our product, but not without avoiding the need to give up something, in this case Mating Conditions, in order to get the maximum benefit out of something new.
Note that we've spread this transition over 3 full releases, NX 5.0, NX 6.0 and NX 7.0, so we think we've taken the correct approach and that our customers should not have any long term concerns once they commit to the move, which of course will not be optional once you move to NX 7.5.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
To an Engineer, the glass is twice as big as it needs to be.