Imagine that you had a perfect single phase transformer. (Primary voltage : secondary voltage ) = (secondary current: primary current) = (turns ratio)
Now take three of these transformers and connect them in a bank as your wye:wye transformer with no primary neutral connection.
For any line to line load, current flows through two legs of the secondary, matched by current flowing on two legs of the primary, no problem. The neutral potential might shift, but the load is not connected to the neutral, so all is well.
Instead consider a line to neutral load on the secondary. Current flowing through one secondary coil has to match current flowing through one primary coil. The other secondary coils have no current flow, so the other primaries have no current flow. But you have a _wye_ primary. Current must at a minimum flow through two primary coils in series. Something has to give, in this case your line-neutral voltage.
What does this matter given a pure three phase load? Only grounding. With a wye:wye transformer and no primary neutral, your secondary neutral can't be used for loads nor for grounding.
There are exceptions to this. For example if you have a three leg core (all three phases wound on three legs of a single steel transformer core) then coupling between phases can make a wye:wye work well enough for grounding. Or a delta 'tertiary' winding can provide the necessary coupling between phases.
Search for the GE manual 'The whys of the wyes' for far more details.
Hope this helps.
-Jon
