Unfortunately in this world of 'i will do what i like and even if i'm wrong i can always sue someone, hopefully my solicitor' solicitors are wise to be cautious.
It may well have formed part of the initial letter/contract of engagement with the solicitor which forms the basis on which the solicitor will act. Law Society states that these indemnities should always be obtained. There are guidance committees that the solicitor could obtain a steer from if they are unsure about their exposure where one beneficiary will not sign. I've certainly come across it before and it is a difficult issue.
I'd be interested in seeing some case law which supports the Law Society guidelines. Every citizen has a statutory right to sue. I've seen Judges go through people for a short cut for forcing people to sign away statutory rights in other situations. Personally I think it is appalling that a solicitor or executor would dictate that someone signs something like this to receive money which is rightfully theirs to receive without restrictions.