Do you think SRST is necessary if the router is configured as CUCME

Nope.  The whole point of SRST is to allow the router to handle call control if the phones can't reach call manager.  CUCME is the router..

Thank you for your fast reply. so, CUCME could have SRST function if we configure it at remote router.
The reason why we use SRST rather than CUCME at remote router is that the SRST is cheap/free, right?

Let me clarify.  You still need to buy an SRST license, but I think you cost will be less than CME.

Thank you!