There are two primary ways to use the Shield.
The conventional one is to use it block heavy weapon attacks, like Gauss shots. Obviously, an enemy can take advantage of this to reduce the shield to pieces with other weapons and save their big guns for other 'Mechs. A would-be headshot turns into a non-issue... that is sure to make the enemy think.
Alternatively, you start using the Shield actively once your armor's starting to get depleted, to absorb critical-hit causing shots.
Either way, Shield-equipped 'Mechs are often good vanguard 'Mechs, something that draws the enemy attention. The Centurion is actually pretty good for this role, since as a medium 'Mech it isn't as important as, say, the Black Knight (which is also a tad slow for the role). A sacrificial piece.
But if the enemy doesn't shoot it, no matter, it gets to melee combat and whacks enemies with the Hatchet and fills holes with SRMs, all to while being nigh impossible to kill thanks to the Shield.