PPC caps are interesting as game mechanic, adding risk-reward and offering additional first-strike firepower. But they need above average pilot and/or to-hit bonuses from AES or TC to be worth it in numbers, IMO. A reg is too likely to waste a charged shot against reasonably mobile or covered opponent.
A 'Mech with single PPC with cap? Sure, it is fine, all it takes is one ton. Charge it up before first attack, then ignore it for rest of the game or until you get a break to charge it, not even too expensive in BV.