well you have to look at this from a holistic viewpoint.  The positives of making this change are... clients are easier to write, encourages people to include 0 fee transactions (in order to reach the minimum size).  The negatives are that it does mean that it's now possible to solve a block, but find that there are not enough transactions on the network to create a valid block, so your work has been wasted.  In addition seeing any kind of arbitrary limit is risky in my view - the benefits of doing this would have to outweigh the disadvantages, and I personally don't see that.

I just don't think making changes to the way the protocol is designed is wise unless there is a seriously compelling reason to.

Will