Justifying upgrading from LDD 2.1i to what version?

You may want to investigate the pricing structure of upgrading LDDT. I'm not familiar with LDDT, but for AutoCAD itself it's become very much a pay me now or pay me later thing.
ie. If upgrading from 2002 to 2004 is $400, then upgrading 2002 to 2005 is $800. (and 2006 when/if it comes out would likely be $1200). So upgrading in steps or all at once basically costs the same anymore (It use to be noticeably cheaper to skip every other version). And of course if you wait too long you will no longer be able to upgrade (You can no longer upgrade 2000i and earlier now). So upgrading to the latest program will give you more time before it is phased out, minimizing the cost of labor/training updating your software one version at a time each year. Though I can understand it being hard to work the cost of the big jump to the latest into the budget.
Ultimately we switched ourselves to the subscription plan. It's been cheaper per year than the upgrade price so far with updates released each year, and it's easier to budget for.
As much as many complain about Autodesk's policies, We have a couple pieces of software that are worse. With this other software, either you are only the subscription/maintenance/support plan, or you will have to purchase the new version at the new price (ie. no upgrades allowed).
-Tim