Is it really worth forcing people to rebase stuff for such a minor change between various bases not designed for being put inside units.
Consistency: being consistent helps players familiarize themselves with the game and is a good way to avoid confusion.
Protection: A powerful, gigantic model should have larger flanks than front facings (like everyone else) to help the enemy flank charge it.
Easy to build: Fitting a large model on a base that's broader than longer is difficult. Like I said in my first post, it only really fits one particular model. Cavalry and large monsters tend to either be quadrupedal or have tails or other appendixes that make it fit a longer base. This is probably the case for 9/10 monsters from any given manufacturer. It should be easy for a player to find models to fit the base size.
Model agnosticism: Given that T9A is model agnostic, we should try to make the design work with a decent range of models from a decent range of manufacturers, not just "that one model from GW".
Now, here's the important part: There are different player bases that need appealing to. For this kind of discussion, I feel that there are two groups with conflicting interests. As you can tell, many of the points I just raised would greatly benefit new players, moreso than veteran players who already have this army. And for veteran players, of course consistency isn't as important because they've already familiarized themselves with it. They don't need an easy to build model because they already have one. I would know, because I'm one of them.
However, when you're making a game, you have to look at the user experience from start to finish. Just because existing players have learned to make and play units that have suboptimal designs - doesn't mean those designs shouldn't be improved on - especially if the payoff means it's easier for new players.
Finally, I'd like to flip the question on its head: Is rebasing one model really that big of a deal? I've rebased plenty. Not too big of a deal and especially if I get a better base in the end. Unless, of course, you've made a really awesome and elaborate base but I'd wager that's a small minority of players.
I would encourage people to think about the unit, army and the game as a whole, about what would be good in a theoretical sense and beyond the particular model that's on your shelf. This is the very purpose of the LAB, to go beyond and improve on what we already have.