Recommendation letters give adcoms a sample of you as a person, academically and or professionally. The "who" is more about proximity to you than prestige or social recognition. Meaning if they don't know you well, then their word is about as useful as a tweet with your name in it. 

I was told once by a GSD professor that their process (at one point, anyway) was to take all the portfolios and divide them into three piles: yes, no, and maybe (or admitted, rejected, and to be revisited). The maybe pile was the only one for which recommendation letters or grades really mattered, because maybe that stuff would put you over the top.

But he specifically said that if you're in that no pile, "not even a letter from Jesus Christ himself" would get you accepted. 

That said, a letter from Glenn Murcutt can't hurt. 

I've always thought it seemed like the personal statement was more important than the letters, especially for people without an arch background.

Portfolio is miles above everything else for people who have arch backgrounds though.

It's important that your recommendation letter be able to say something about you as a person.  It is useless to have a reference from a 'big name' if they can't say much about you as a person, your character, your work, your ambitions, etc.  Obviously you want someone who knows you in a professional or academic context, a letter from your mother isn't going to help.  However, the best letters I've read on applications have come from colleagues and mentors who knew the candidate well and could make a persuasive argument for WHY they thought they would be a good choice for our school.  A 'Yes, so and so worked for us for 3 years and did a good job' letter, really isn't helpful in making a decision about a candidate.