Good points.
To answer the question:
Tensiometer can tell you the absolute spoke tension
(while plucking the spokes basically tells you only their relative tensions between each other).
For a novice builder, I suppose that knowing the absolute tension can help prevent building wheels that are under or over-tensioned (even if the tension is uniform).
Yes, I suppose you could calculate the absolute tension based on the spoke length, thickness, material, and measuring frequency, but I think using a tensiometer is simpler.
A solution to that problem, without using a tensiometer (and with the relatively thin bicycle spokes), is to rely on hand feel - and check what a wheel that you know is well-tensioned feels like (its spokes that is).
This has worked well for me in practice, so I would say that it works.
Note:
With any "exotic" wheels (carbon, low spoke count, "aero" spokes etc.) it gets even trickier to work without a tensiometer (it can be expensive if you overdo the spoke tension and the expensive "plastic" rim cracks).
A digression:
Having said that, for me at least, it is easier and faster to work with a tensiometer, compared to using a pluck, even with "normal" aluminium rims.
That is problably because I am practically incapable of discerning different tones (my hearing is fine, but playing or singing... nope
).
However, based on the feedback from "normal" people (and a couple of musicians
), pluck can be more accurate than using a "standard" tension meter (the likes sold by DT Swiss, Park Tool and the others) - but, again, that doesn't really measure the absolute tension.
So, you could build a good wheel without a tensiometer, but it might be a bit faster and easier if you use it.
Relja