My Seamaster reels have no holes in the spools; and I can't say I ever discovered any weaknesses in them as a result.
I particularly despise holes in the spool arbor. Very little weight and moment of inertia reduction, results from boring holes in an (existing) arbor. And it is trivial beyond belief, to make that up, by simply turning off a few more thou, from the outer surface of the arbor.
I'd really like to see an actual structural analysis paper, that proved that a spool with holes, can be made more rigid, than a no holes spool of the same dimensions, and exact same weight.
And that is giving zero consideration to the minor fact, that you actually want to put line on this spool.
The side walls of a reel spool are a cantilevered beam. Literally, a 360 degree panoramic diving board. The wall thickness should taper, from the arbor to the edge, being thinnest at the edge. The stiffness of a uniform thickness spool wall, increases towards the rim, as a result of the increased perimeter.
Seat of the pants says the stiffness of a diving board increases linearly with the width. A four foot wide diving board would be twice as stiff as a two foot wide one, all else being equal; so the bending stiffness of the spool wall should also go linear with the diameter. But the edge of the spool doesn't need to be as stiff as the inner part of the wall, so the rim can be thinned, relative to the hub, just like the diving board.
A proof that holes can increase total spool wall stiffness, over no holes, for the exact same weight, is something I would like to see; but I ain't holding my breath.
Holey spools sure do look pretty. Mako spools look like works of art.