[racket] feedback on migrating to immutable pairs?

Two questions on the move to immutable pairs:

1. Does anyone have feedback on how that went?  (The only problems I 
recall dealing with personally were pain for HtmlPrag, a minor change to 
the "csv" library, and a small headache supporting SICP.)

2. What was the rationale for not having "pair?", "car", "cdr", work on 
both mutable pairs and immutable pairs?  Was this to push everyone 
harder towards immutable pairs, at the cost of having to convert legacy 
libraries and breaking R5RS compatibility?

I'm asking because this might be valuable input to a Scheme working 
group.  Thanks.

-- 
http://www.neilvandyke.org/