That's true, but do you care? Like I said is this a one-off or a production run of 1000s. :) It make a difference. If it's running off batteies that makes a difference as well.
To down shift all you need is two resistors. To upshift is harder but if the Xbee puts out 3v3 then that should work directly with a 5v Arduino because the logic high threshold of the AVR chip is 3v.
Not exactly production strength but should work.
You can upshift with a transistor and an inverter or two transistors or a comparator or an opamp, there's no end of options but I reckon those TXB chips have to be the simplest way to go. There's a 1-bit version as well (TXB0101), use resistors for the down shift and the TXB0101 for the up.
The Adafruit board is cheap but at $10 it will add maybe $20-30 to a finished product. You could prototype with that then do a final board.
Too many options eh?