Router vs. firewalls at Internet edge

I’m looking at a design for an Internet connection consisting of a Cisco 2921 price  router as the Internet hand-off device (Ethernet), with an HA pair of ASA 5525-X firewalls sitting behind it. I’m looking for any justification whatsoever for why we’d need to have that ISR in front, vs. just Ethernet into a local Layer 2 switch feeding the ASA HA pair.  I know that we could accomplish the firewall piece with the ISR Itself (using the Security license), but the ASAs are also providing IPS services. Plus, the ASAs were sized based on their IPS capacity (which is documented at up to 600-Mbps).  Having said all of that:

The 2921 is generally positioned as being able to support 50-75-Mbps WAN throughput with services (far below the capacity of these firewalls). I know it has GigE ports in it, but they do not provide line rate performance.  The 2921 build they’ve provided does include an EHWIC-4ESG, which provides line rate Gigabit performance at Layer 2, so that could be used to handle the incoming handoff and switching to the firewalls. However, the router build does also include the Security license, so clearly it was being positioned as having some role in securing the connection.  (Again, if all they needed was a Gigabit switch to front the ASAs, they could find a lot less expensive option than a 2921 w/ add-on EHWIC module.)

Assuming that the Internet connection is being provided via Ethernet (as opposed to a T1/DS3, etc., that would require a router to terminate), is there any other GOOD reason for having a router front the ASA pair?  I can’t see one, and it’d represent a serious bottleneck anyway.

Although both can do each other's job, but result often becomes same if you make a singer play football. In small env, it does not matter, but in larger throughputs, it becomes difficult.

You must be knowing what will be the throughput to your internet and accordingly you can select the device which will support that. Else, there will be drops.

If you need basic FW features, then 2900 would be fine compared to the cost involved in ASA's. Often people deploy firewall and it ends up in a overkill. Purchase only that much which you need.

Please note down the features you are looking for in routing (like bgp or internal routing) and security features you are looking for. Then just tally it with $$$ worthiness and you can add/drop features required and your hardware selection becomes easy.

Please keep in view of the scalability i.e what will be your requirement in the next 5 yrs and there should be some investment protection while purchasing e.g they do not have BGP now but what happens if they want to have their own AS and manipulate routes 3 yrs down the line or add up a few offices which need routing?

Please note there is nothing called a best design. It just needs to be technically correct and can be deployed in any manner. The sole factor driving designs is Cisco 2951  and business requirement.