WAP (Web Application Proxy) currently only supports "pass-through" and "preauthentication" as authentication modes, which fits lots applications, but not all.
When there's an application, which uses Kerberos and has public facing sites, there's no possibility to publish the application via WAP, thus it would be advantageous to have an "on-application-demand" authentication mode, which will authenticate a client, when the application sends HTTP status code 401.

Application Request Routing (ARR) provides a good proxy experience and load balancing features.
Web Application Proxy (WAP) provides the possibility of authenticating users and map the authentication to Kerberos. It also provides the AD FS Proxy functionality.
WAPs proxy features are very limited in comparison to the proxy features of ARR. So ARR features should be available in WAP or since ARR is hosted on IIS WAP features should be available in ARR and thus in IIS.