Yes rear declaws are not a deformity, many many breeds have them and are actually required to have them a part of the breed standard. In shelties the breed standard requires the removal of the rear declaws if present(so any breeder who leaves rear dewclaws doesn't not know the standard). SO many shelties probably had them but the puppy buyers don't know it.
All of my dogs(the 3 related ones) except for Birch had rear declaws at birth. Justice actually had double rears. All removed at 2 days. Genetics play a role. Front dewclaws removal is breeder options.
We do not remove front dewclaws because as Linda said they have a function.
http://www.caninesports.com/uploads/1/5/3/1/15319800/dewclawexplanation_rev_apr_10_2013.pdf
As for rear in an adult. Depends how floppy, how active is the dogs , how tight is the dewclaws, how easily can you keep the nails trimmed close. It is still a surgery which will require aftercare and is not as simple as when they are 2-3 days old. My thought is if they are not a problem, leave them alone. If they become a problem do something about them. I find if owners keep the nails trimmed close , they are not a problem. I can count on 1 hand the number of dewclaw injuries I saw in practice in dogs with
PROPERLY trimmed nails.