The reforms started earlier when the empire was at the point of collapse and for me they make good sense, much more then the augustan model. The principate army was placed in a thin line along the borders, often to thin to stop a large invader striking a point and as there was no strategic reserve the romans had to pull other units from the border, gather them in an area and then chase the enemy. In the process the border areas were laid waste. The reforms created a larger, politically more reliable and cheaper force that defended better constructed fortifications along the borders - the limitanei, while field armies acting as highly mobile strategic reserve were concentrated under the watchful eye of an important official (sometimes even an august or caesar) ready to move and meet the enemy.
The military, political and religious reforms of that period prove that the empire was still capable to reform itself in order to survive.