I recall some discussion quite a while back in time -- more than two or three years --, where somebody of PG Staff said something like: "We don't mind a second installation if it is the same user." The wording was definitely different and acknowledging that one is working on a desktop at home, but away from home, like on a stage or in a hotel room, it might be used by the same individual on a second computer. Sometimes it is inconvenient to carry an extra external disk, like if you're working on battery these things consume quite some time's worth of energy.
that makes sense, and it would be in PGMusic's advantage to allow this on the same person's machines.
Having said that, it would be less confusing if the very first line in the EULA did not say something that is intuitively interpreted to mean something else. (I'd be surprised if their lawyers would endorse the idea of changing the EULA's wording)
Getting back to the idea of 3 installations:
lets say you have BIAB/RB installed on 3 different computers, because you can. If any one of them needs to be replaced, you've already used up your 3 installations. Hard drive failures don't usually give you the opportunity to unregister software first. Technically, you'd have to purchase another 3 installations. For the person who upgrades every year, probably not a big deal.. but the likelihood of this scenario happening increases once you skip an upgrade and you're trying to keep 3 installations going as long as possible without spending new money.