I see no reason for regenerator not to be the primary slash on every set.
The prevalence of SR, and the difficulty of spinning make regenerator an option with much, much more general utility over pressure. The ability to switch in many more times, and even be able to come in on certain things with enough health to recover and begin the devastating cycle of sub attacking.
Pressure has some niche uses to be sure, like pp stalling certain attacks. But to be able to do this versus a competent player (e.g. one who doesnt switch in palkia to attempt to break a ho-oh already on the field, or one who knows not to run stone edge support groudon without roar) SR generally has to be off the field if you choose to use pressure.
I don't think the utility of stalling out a few moves, which you can generally do anyways with smart subbing and the added recovery regenerator brings, outweighs the ability for ho-oh to avoid losing to some of its main counters, like SR, and bulky phazers while sr is present, including lugia (if ho-oh is behind a sub) and most importantly gira-a.
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