Even aside from the fact that this "diorama" would have looked a LOT better with a decent base, SS let us almost "count" on some of the surroundings, or at least a recognisable base, based upon the diorama's we've already gotten from them in the past.
Apart from the very first (which I, again, wouldn't call a diorama as practically nothing of the surroundings of Mt. Doom was included) and the Eagle Vs. Fell Beast, all diorama's from Lord of the Rings had at least some surroundings/environment in them. Especially with the pricetag this dio had, (almost) everybody was expecting at least something that made it recognisable that this was on the top of Weathertop. When it's time to pre-order the thing and you see absolutely nothing like that, people are (rightfully) disappointed.
Going from: "Cool, I'm excited to buy this! I can't wait to see what it looks like!" to "Holy ****! Is this ALL??" in just a matter of minutes. It looks ridiculous seperately. Period. The Ringwraiths look cool, Frodo looks awesome, but the seperation between the two bases is complete nonsense, and by some people purely justified by saying "Otherwise SS wouldn't be able to sell the regular version as the base would be half empty" (i.e. Frodo not there).
Frodo shouldn't have been a "companion piece", it should have been included in the regular version.
Just my opinion, of course.