One of the many things that my customer ask why we donot go for layer 3 switch instead of router when ethernet is our only requirement. Cisco 1 port FE card still have GPL of 950 USD. While L3 switches comes with 24/48 port and this is really cheaper when customer require many Ethernet interfaces.
Well although in some cases the customer may opt the option but i still don't encourage them as one obvious answer is router is still intended for routing while L3 switch is still have switching hardware and intended for LAN . It cannot support many necessary features like IPSEC VPN, GRE tunnel and even NAT that are necessary for WAN edge
Well i still remember from my previous job experience that one of my colleague purchase 3560 for site to site vpn with router, only to figure out that IPsec is not supported on it.
To summarize L3 switches were introduced by keeping mind to perform efficient intervlan routing and are best within campus. Routers are destined for edge and will still be used at WAN edges.