HTC were probably right. This is going to get really messy for non-rooted stock users.
A Fresh install of stock froyo with HTC Sense on a Desire leaves you with about 140meg free to install apps. This doesn't go far - install a few big apps like google earth and that soon disappears. Of course, you can move some apps so SD using the Froyo method, but most people don't know this so won't bother.
So what happens when you try to push an even bigger version of Sense to them? The install will fail, users confused and annoyed, wondering where they can't update thier phone or have to delete their apps to get it working again.
Of course, Rooters and Modders already know that you can install App2Sd or Data2Wherever - right now I have MIUI running with 1GB of interal memory using an EXT 3 Partion - but that involves - rooting the phone, partitioning a card correctly, downloading and installing a rom - not really ideal for non techys eh?