While in most normal battles, and many early boss battles, heavy offense might work okay, most boss battles and specialty battles (like the big game at later levels) are tuned around the use of the sentinel.
The first battle where you can see this in action is Gogmagog in New Bodhum -003 AF-. If you haven't been grinding CP and you just stick to Slash & Burn (COM, RAV) Assault or Double Trouble (COM, COM), the battle is doable, but it's hectic. Your HP will dip down significantly every time he uses his "Swipe" ability, and if you're not fast enough, you might even need to potion up.
But, if you take the half a second to switch to Twin Shields (SEN, SEN) as his "Swipe" ability lands, most of the damage is negated. Switch back right after the damage numbers come up and you're back in business. If you juggle properly, the battle is much easier and you'll still get a 5-star rating.
This same strategy will play out in many, if not all, boss battles: a hectic battle with lots of potting (or even retries) becomes much easier if you time a defensive paradigm shift right when the boss is about to unload.
The key is that the paradigm system is meant to be used all the time in battle: unless you're grinding, you should be paradigm shifting in a battle a lot to maximize your strengths and minimize your weaknesses. Switching to a defensive paradigm for a few seconds will always be much more efficient than trying to recover after a damaging attack by switching to a healing paradigm after the fact.
So while you likely don't need a dedicated sentinel, being able to switch to a paradigm just as a really devastating attack hits is a big deal and can make or break a battle, particularly in the later stages of the game. Keep a monster with SEN in your paradigm pack, and have Tortoise (SEN, SEN, SEN) in your paradigm deck for those "oh crap" moments.
And even in your healing paradigms, taking one slot for a sentinel is not a bad idea: there are only a few things immune to "Provoke", and a sentinel can keep itself alive with "Mediguard" (for when the sentinel starts out at an HP deficit) and "Steelguard" (for when it doesn't).
If you do add a sentinel to your healing paradigms, Combat Clinic (MED, MED, SEN) makes the enemy focus on the sentinel while the other allies heal up, while Solidarity (COM, SEN, MED) does the same thing, but lets one character focus on maintaining the chain gauge while you recover.