A PID-controlled unit will not have a quick enough response to have significant effects in a system with a large enough heat sink and boiler. For something like a Gaggia it is absolutely indispensible, but for a larger double-boiler unit with a huge brass grouphead it becomes just another link to break. If you have a huge, intermittent, throughput it might be useful, but I'd wager that even then we'd see far more variability introduced by minor loading and tamping inconsistencies than by the small heat perturbations seen with a robust semi-commercial double boiler unit.