Use of 70 oz Hydration Pack during Ironman

advantages: you are comfortable with it, always have water
cons: maybe uncomfortable? heavy.

you don't NEED it -- there will be plenty of aid stations on the bike

Training?  Do whatever you need.

Racing?  Why bother????

You should be able to get around 140 oz on the frame without using a hydration pack.  On an IM you should only need 1 water bottle and 1-2 nutrition bottles unless the temps are over 95F.  You could go with a Neverreach if you want additional security water wise without the extra effort of carrying that monkey on your back 8).

I used one for a HIM once.  Never again.  I just didn't see the point.  Unless you have nutritional needs that can't be met on course, forget it.

They have aid stations every 10 miles or so, so you can load up a bottle on the fly all the time and not lose any time.  Plus save the weight.  (and on my rolling to hilly courses, that's important)