This is a somewhat trickly situation for a number of reasons.

From the Dev's perspective I think the main concern is gameplay requirements (which in turn means barriers to access on content), and content balance, particularly on Hardest level content.

When a character hits 60 now and wants to move into raiding the key obstacles is to get some gear with enough ilvl and find a group. Over all its not too bad. You have many options to get that gear and generally regardless of the source, if its the right ilvl it will put you in a situation where differences in stat allocation on the gear wont make or break performance. However the more difference those stats make, and the more complex and varied their impact on gameplay is, the larger the difference specific sets of gear will have on performance. The percentage difference in performance between two sets of the same ilvl with different stat allocations suddenly substantially more.

This is the evitable 'best' build which becomes the goal of most players. For a balance perspective the dev needs to take into account the strength of such a build because if they don't the Hardcore content suddenly becomes a lot less hardcore. However doing this means that any player not with the 'best' set will find the content noticeably more difficult, pushing the 'best' set into being mandatory to do the content.

This then starts to undermine the value of that customisation option. Its no longer a choice because you have to play optimally to do that content. Further it creates a itemisation and reward issue. If there is now mandatory gear for raiders it means accessing that gear has to be something everyone who wants to raid has to do. Suddenly any other way of earning gear drastically loses value apart from glamour, greatly reducing player interest in these alternative activities. Raiders will then have to do whatever method is needed to access that 'best' gear even if it is a gameplay style they don't particularly enjoy. More over it means that any rewards for content that the gear is needed for have to be powerful enough that any loss due to stat optimisation being effected by the new gear (such as perhaps the loss of a particular bonus effect on an ability) is mitigated otherwise the rewards for doing the Hardcore content will lose value and peoples interest in actually beating the content will drop.

Its a trickly problem and understandably one they aren't keen on tackling. Does it mean I don't think they can do anything? Nah. There are options. However any such system would need to be very carefully implemented and noticeably limited in scope to limit the impact such variation will actually have.