Thanks for the idea.
The major problem with the logic that splits an order into parcels is that it is a resource hog. The more items a customer has in the cart, the more combinations the cart has to look through to find the one that works better. Having 1000 items in the cart may put your website down (don't think of ordinary customers, think of hackers trying to down your store). That's why we added the easier alternative.
If we find a way to get over the performance problem, we will surely add the auto-split logic.
By the way, you can vote for the idea at
http://ideas.x-cart.com/. Although we don't use the number of votes as the priority when choosing features to implement, we do consider it when deciding such a priority.