Not really lesser, just lacking the refinements--and possibly even standardization, like magazines--that the newer ones have.
The MkIII is indeed drilled and tapped out of the box, or at least my 22/45 Target is. I think the MkII depended on model, but I can't promise.
The MkII was also adopted more commonly and thus has more aftermarket support, where the dimensions for parts for the MkI may be slightly different, and the MkIII can use MkII parts with the addition of a spacer in place of of the mag safety.
Also, in some cases, the MkIII may be a hand up for new shooters. It seems easier to train them to always drop the mag (thus engaging a mechanical safety) and check the loaded chamber indicator before turning away than to train out the muscle memory of their grip on it, but it's also an extra reminder to teach them to check their trigger discipline.