RandomAccessRange / Mobile Elements

[Johann via Digitalmars-d-learn]

Hello,
I read in std.range that given a random access range r , 
r.opIndex(n) should return a reference to the nth element. Is 
there a qualifier for a "read only" random access range? If not, 
why?
I also don't really get the point of "Mobile Elements", how is 
"destructively reading" related to mobility, and what is it good 
for?

Thanks.