Um, can someone please explain to me why we think there were regents pre-Deismaar?
I don't think there is an argument that there were regents in the sense of blooded regents. As you say, though, there were certainly rulers. The problem with the RAW was that any action that requires the use of RP to even attempt is an action that by definition is denied to someone without RP. That's a problem when you're trying to conceptualize rulership before bloodlines.

Even if no one is interested in playing a pre-Deismaar game, or even of running a domain without a bloodline, I think it's a worthwhile thing to address for game continuity. Note that portability to other settings can retain RP intact without bloodlines, substituting some other mechanism in their place.

I think it's most important when considering the elves. While humans and dwarves would see bloodlines as a blessing and have that much more cultural pressure to ensure that their rulers were blooded regents, elves might more reasonably actually look upon a blooded elf as accursed and share no cultural preference for one to rule. By this reasoning (more consistent with the original concept of the Sidhelien, I think), the rules should not make it impossible for a non-blooded elf to govern, and perhaps shouldn't even really put him at too great a disadvantage as compared to a blooded regent.