It may seem odd to disband them but think of it this way...

If you disband them in City A they will be added to the population of city A and then raise a unit of Legionary troops in that same city the population will be deducted again... So you are not really lossing any people when you do this!

You notice that you lose the ability to even retrain the pre-marian units after the reforms so you can not replace losses in an out of date unit?

So they really have become redundant in the face of the army reforms and whether you simply disband them to add to the population of your cities and send the off in a gorious and bloody battle is up to you...

It better than still having your old FAA running around in the late medieval period kicking arse because they have loads of experience (like 300 years!) and have been re-equiped with the latest weapons...

PS. Total agree on the reforms happening 100 years or so too early... Forgetting the historical outrage it is really poor gameplay wise as you have to fight hard to get Triarii but just as they arrive they are out of date!!!