I have never met an adoptive parent who regreted having such a meeting. Most people find that, as Devora said, it makes the birth parent/s become human and real, rather than just writing on a document
I personally have met DD2 and DS mum several times. It's enlightening meeting someone in the flesh. I have a much better sense of who their mother is from a meeting than I could get from the information I was given on her beforehand
I also value the conversations we have had, and I can relay them to the children and know that it's fully accurate. I can tell them about her from my perspective, rather than the perspective of the social worker. I could answer some of their mothers questions. Reassure her that I will love them and care for them. To her, I also became more human I guess!
And as Devora said, photos of you together can show your child that their birth parents are at least somewhat accepting of the situation. I know some adopters (me included for our first meeting) went into the meeting apprehensive, and finished it with a hug!
My LA say publically on their website that they expect all their adopters to agree to a one off meeting (if appropriate). It's a condition of being accepted and progressing to homestudy. That might mean I know more people who've had meetings than maybe elsewhere in the country, if other LA's do not do these meetings as frequently?