A couple of disadvantages.

1) On a slope, the luggage is likely to roll by itself (this happened in our driveway).

2) Higher probability that one of the wheels will be knocked off. Two wheels use in-line skate design for the most part, where as the spinner wheels protrude - this can break them more easily

3) Harder to balance - if one of the wheels is out of balance, it can throw off the luggage.

I recently borrowed a piece from one of the family members for a weekend and I was not too impressed with the advantages of a spinner.