When ARRv2.5 was released in March 2011, it was made available only via WebPI.   (Release announcement is here and the explanation on the WebPI dependency is here.) 

Since then, I've been asked if ARRv2.5 can be installed outside of WebPI, specifically for those who want to install ARR on Server Core where WebPI is not available.  In short, yes, ARR can be installed without WebPI.  But before we dive into that, let me provide more contexts.  ARR consists of 4 IIS Extensions and WebPI manages the dependencies so that IIS Extensions that make up ARR can be installed in a correct order.   The 4 IIS Extensions are:

• URL rewrite
• Web Farm Framework
• ARR
• External Cache

So to install ARRv2.5 (only English version is available at this time) without WebPI, you would have to install the 4 IIS Extensions separately in the following order:  

Stop IIS first by typing "net stop was /y" and "net stop wmsvc /y" on an elevated command-line window.

• x86:
• URL rewrite (v2): Download from here.
• Web Farm Framework (v1): Download from here.
• ARR (v2.5): Download from here.
• External Cache (v1): Download from here.

• x64:
• URL rewrite (v2): Download from here.
• Web Farm Framework (v1): Download from here.
• ARR (v2.5): Download from here.
• External Cache (v1): Download from here.

Don't forget to start IIS and you are all set.