Object Desktop gives you the full versions of many of the programs you now use, at a discount - compared to the cost of purchasing WindowBlinds, IconPackager, and DesktopX individually.
It also includes a variety of other programs that you are not using, which would add to the over-all cost of the individual programs (i.e., SkinStudio Pro, IconDeveloper Pro, Right-click full verision, etc.).
ThemeManager is only a co-ordinator of files and programs. By itself, it cannot do anything without the programs which actually apply the skins or themes (WindowBlinds, IconPackager, DesktopX, etc.).
As for Object Desktop offering something you can not already do, I would say that skinning the explorer shell requires one of the following:
1 - Third-party program - WindowBlinds, NextStep, or alternative shells. There are shareware, freeware, and full versions of many products - and the full version (or freeware) will offer a more comprehensive application of the skin, theme, or shell.
2 - The ability to write code to create a skin that is compatible with the "Themes" service, or the use of something like StyleXP which helps hack the shell and enables skinning.
3 - A program like ResHack and a lot of time needed to play around with hacking files to tweak the explorer shell, which would probably not be complete anyway.
To change icons, you would need one of the following:
1 - Third-party program such as IconPackager, which will change many of the system icons, folder and drive icons, and as many file type icons as you like - all with minimal effort other than combining a package of icons.
2 - a hacking utility (again ResHack might help) to open and hack files such as "explorer.exe" and "Shell32.dll" to change your icon images and perhaps some work in the registry, depending on how you want to name and assign icons - all of which would take significant amounts of time on top of combining icons to create your files.
As I see it, the decision to purchase Object Desktop would be based on whether you intend to use the full versions of the programs included, and whether they would cost more individually than purchasing them as a discounted package with Object Desktop.
The shareware versions are just that, "shareware" - and some are already "30 Day Trials", so obviously this avenue is short lived.
BTW - I use WindowBlinds, IconPackager, and ObjectDock Plus on a regular basis - and now that I have seen the promise shown in WindowBlinds 5 beta, I will seldom see the "Luna" shell style applied - other than to create tutorials and perform certain tasks where all programs must be unloaded to eliminate any anomalies in test results. 
