I looked at something similar, a MAX3110/3111 SPI UART. After pricing a basic configuration, I realized I could use an ATmega for half the cost, using parts I already have on hand, and have almost 1k of buffer space instead of a few bytes.
If the UART peripherals provide some special service you really need they may be worth it, but if you just need a basic UART, rolling your own with an ATmega may be the cheapest and simplest.
-j