Google has taken things to a new level with Ice Cream Sandwich, and now they are extending that to additional support for developers. In the past, the fact that multiple OEMs create different add-on UIs to their products has been a source of frustration for developers. Their apps could work fine on a stock Android build, but might get broken by one of the different manufacturer UI overlays, like Motoblur, Sense, etc. Now, instead of having to develop multiple versions for the different UI skins, Google is requiring OEMs to include the original Holo ICS Theme with the device. This should make things easier for devs moving forward, because now they have tighter and more focused standards to follow to make sure their product can work with multiple ICS devices. Here's a quote from the Google Developers Blog:
You can read more details on Google's developer blog below.In Android 4.0, Holo is different. We’ve made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward. If the device has Android Market it will have the Holo themes as they were originally designed.
This standardization goes for all of the public Holo widget styles as well. The Widget.Holo styles will be stable from device to device, safe for use as parent styles for incremental customizations within your app.
Source: Google Android Developers Blog