Each Ubuntu derivative has its own "desktop". For example,

• Unity has the ubuntu-desktop


• Lubuntu has the kubuntu-desktop


• Kubuntu has the kubuntu-desktop and


• Xubuntu has the xubuntu-desktop

Each of these "desktops" is a metapackage, decribed in detail in MetaPackages. These metapackages contain nothing but a list of software that give you the corresponding desktop experience. Installing lubuntu-desktop, for example, on Unity, will pull in all the software associated with Lubuntu and afford you the choice of running either Unity or Lubuntu at the time of logging in.

There are times, when, as you have observed, that you want to delete a piece of software for whatever reason and you maybe be warned that doing so will remove ubuntu-desktop (or some other desktop depending on your environment.)

In fact, Synaptic has this caution regarding the ubuntu-desktop:

This package depends on all of the packages in the Ubuntu desktop
system

It is also used to help ensure proper upgrades, so it is recommended
that it not be removed.

Based on the Synaptic comment, removing the ubuntu-desktop now may be detrimental at some future date.

On the other hand, Synaptic says the same thing about lubuntu-desktop whereas community documentation on Lubuntu assures Lubuntu users that there's nothing to fear.

When you want to remove a program from lubuntu, you may be told that
by doing so it will remove lubuntu-desktop. The package
lubuntu-desktop is only used for the installation of lubuntu and can
safely be removed.

Lubuntu-desktop is what is called a meta-package which, simply put, is
just a list of what to install. It is a design decision that all
packages included in lubuntu-desktop depend on it, so as to be able to
use --no-install-recommends option when the initial installation
occurs, thus preventing non-essential programs being installed.

The lubuntu-desktop would just need adding back in for the update from
10.04 to 10.10, after that it can be safely removed again.

Even though the quote above refers to 10.04 and 10.10, more recent posts elsewhere have similar opinions. You could look at [Lubuntu-desktop] lubuntu-desktop metapackage - Recommends vs Dependencies and the responses. Again, IMO, even though the desktop being talked about is Lubuntu, the same observations apply to Unity.

In my case, I no longer have lubuntu-desktop because I chose to remove a package of games, aces-of-penguins, and lubuntu-desktop went with it. I must mention that I will be doing a clean install from 12.10 to 13.04 and not an upgrade.