First let me point out that the cart option does NOT require that users be logged in to have any field auto-population. Non-logged in users, when they click to go register for another event, will have fields auto-populate from the registration that is already in the cart.
Second, while we have this cart type feature, DT Register is NOT a shopping cart. Due to the highly customizable elements of each event and the fact that every event can potentially have a completely different set of fields, pricing, etc... we can not just setup this cart to allow registrants to simply throw events into a basket, applying one registration form to all of them.
While this would work for some sites, it would not for most. This is an EXTREMELY complex thing that has already been discussed in other forum threads. We are looking at some possible enhancements to allow this type of functionality, but it will require a large amount of work... work that we haven't had the available man hours to tackle yet. With the large amount of moving parts in this extension, something like this will require changes to many areas, including some structural changes. It may come in phases. How soon? No idea at this point.
Just know that we are aware that it is an area that can be improved... and know that it's more involved than one might think. We'll get to it as soon as we can... unless someone jumps in to pay for such customization work getting it on the table sooner than later.