Frequent assumption from customers: our monitoring tool is SCOM so we do not need SPHA. As a result customers do not pay attention to configuration of SPHA and do not take advantage of it.

Actually this is wrong. SCOM with SharePoint management pack monitors not only servers, but also rules defined in SPHA (http://technet.microsoft.com/en-us/library/ff686816(v=office.15).aspx). And these rules do not exist in SCOM management pack for SharePoint by itself. So by using SCOM you still using SPHA and it provides useful insights into health of the deployment. Moreover rules in SPHA can be customized, so it can be seen as lightweight way to customize SCOM monitoring of SharePoint. So:

  1. Configure diagnostics.
  2. Configure SPHA.
  3. Install SCOM management pack.

Sėkmės! / Good luck!