To workaround this issue, I use the require upon moving to a new status instead of current status. It works very well, although you may end up have transitory statuses (statuses used to just require a specific grouping of fields or to support fields that should be required upon transition, but view only after the transition is complete). I have been working with PPM/ITG/Kintana since v5
As far as I know Draft mode is only available for new requests, not in the middle of the workflow; also, PPM Admins in charge of workflow configuration may not realize when getting the value of a mandatory field will require to make a phone call to a different person and eventually having to wait in the middle of filling the fields of the request - so saving the work-in-progress without mandatory fields would definitely be helpful.
It should be possible to save a request without having all mandatory fields entered.
It's possible to do it while the request is not yet created (draft), but once it's submitted, you cannot save it while mandatory fields are left empty, which leads to loss of work when it takes a long time to get the value of a given field.
Current design justification is that we want to make sure that fields flagged as mandatory always have a value in the System, for reporting purposes mostly. Saving a request without some mandatory fields (even if you don't try to move to the next step) might create new problems for UI rules - this is a risky change, which was already tried in the 9.2X days but failed to work properly and was rolled back eventually.