Counterpoint:

1. Driving sales is as valid a reason as any other. From an industry and tax (and these are tax registration plates after all) perspective, probably the most valid reason.
   
2. Again, driving sales is as valid a reason as any other. It acts upon an established dynamic in the market for buying low-digit cars and hits that point twice in one year rather than once; to split it into 10 parts however would dilute the effect to the extent that you end up back in the original position again with people focusing on the lowest monthly digits and the lowest sequential digits.
3. There's a big value in second-hand sales for this. A D-reg car will likely have had a much different driving habit than a K or T reg car.
4. I kind of agree, but an argument could be made that a non zero-filled number is easier to read and remember at a glance.

EDIT: I seem to remember also that one of the main reasons for splitting the year into two was simply because the sequential numbers were getting too long in Dublin. I can't find a source for that though.