Re: Somone think of the ARP table!
You've eliminated the large broadcast domain by routing, but you've increased the size of the ARP table. You now need a device with larger ARP tables than you used to with 3 tiers, which current cheap silicon doesn't have, since it was built expecting to need only enough ARP entries for a 3 tier application. (And typically ARP tables are replicated across a stack, so they don't scale up).