I am sure that there are thousands of games out there that do not use motion capture, and yet they still possess amazingly lifelike characters.
Take Elizabeth from the recently released BioShock Infinite as an example. She is played by three different woman. One did the motion capture, one did the facial expressions, while the third did the voice. And the character has received a lot of praise for her amazing character and lifelike nature.
So, why must Garrett be played by only one actor? With all due respect to Romano Orzari, there is only one voice actor who can do Garrett, and we all know his name.
We've never made a big deal out of this because we thought Eidos knew what it was doing. We were promised a Thief game, which means Stephen Russell went without saying. But suddenly, they decided to change him because he cannot do motion capture.
Russell is a talented and a professional voice actor, he knows how to do a voice depending on what the character is doing.