Ray - we’re talking slotted bridges here, not slotted saddles.

Major builders such as Martin and Gibson used to slot the pin-holes in the bridge, and use solid (un-slotted) pins. Some high-end modern builders still use that practice - Collings, for instance, and the Martin ‘Authentic’ Series still have slotted bridges and un-slotted pins.

Some time in the mid-‘40s, Martin stopped slotting the pin-holes and started using slotted pins. It’s difficult to find the reason set out anywhere, but it’s generally accepted that, as the company moved to ever-greater production numbers, slotted pins were cheaper than slotting the bridge, and therefore they chose to go with them in the name of economy (and thus more profit).

The benefits of slotted bridges with un-slotted pins include…

1) A more solid contact between the ball-end of the string and the bridgeplate
2) because of 1), Improved tuning stability
3) the tendency of the ball ends to ‘creep’ into the pin-holes is greatly reduced, or even completely removed
4) reduced wear on the bridge-plate/pin holes (because of 1 and 3)
5) improved break-angle when the saddle is low (although this can also be achieved by ramping the top of the hole rather than full slotting)

There may be other advantages, but those are the main ones.