That may be how it is in 4e (haven't played it, don't know), but that's a flawed assumption from the get go. It's only true if you recover all resources after every battle. And that's simply not true in most gaming scenarios. If you're given the choice of ending an encounter in 4 rounds with 55 HP left, or in 3 rounds with 50 HP left, the first is by far the better choice because you might need those HP later on.
This example completely ignores the value of ending the fight one full round earlier. That is, it sets a zero value on whatever nastiness the monsters would have done on round four.
In reality, chances are that nastiness is much worse than you losing 5 extra hit points. For example, the monster eating the face of the squishy wizard or bard for 6 damage.
6 > 5, thus making the offensive option better.
Focussing on your offensive instead of your defensive is generally a good idea in a game like D&D.
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There are exceptions, such as the tempest Cleric with Spirit Guardians mentioned above.
Our party has such a Cleric. When things become really hairy he spends his action Dodging, letting the spirit guardians do the job for him. Cast at 5th level, they do a pretty good job.
He has all the feats and a great Constitution, so DC 10 Concentration checks are an auto-success for him.
In this special case, it would be madness to not go for a shield.
(To be honest, this isn't really an exception, since it is yet another way of maximizing offense. Since there aren't any real tanking possibility in this game, including aggro mechanics that govern the actions of NPCs, it is almost always substandard to go for a defensive build, when you can go for an offensive build and still get decent defense. Aggression is definitely the name of this game.)