Hi Scream
Well, I do it very regluarly for my youngest two. We have a more open situation than most, with photographs, videos, cards, presents and meetings as well as letters. I write to their first mum and to their brothers and sisters - I am in contact with 14 siblings and half siblings atm, and one aunt and uncle. Although quite alot of the siblings are living together and I'm not writing seperately to all of them!! I do wind up writing quite a lot though and i have to be organised to get it done!
It's something to think about because it's probably the most common arrangement there is. Most people write about 1 or 2 letters a year, but if there are siblings you may well end up with more. The majority of children avaialble have letterbox, and if you didn't want it you would be quite restricted on which children you could adopt
People have pretty varied experiences. I tried with my eldest, but she didn't want any contact and I was happy to stop tbh. It was just a reminder of being abused. But it has been very positive with my younger ones, in very different circumstances.
My children benefit in various ways. DD2 is close to her mum, and always reassured to know when she is doing well - she was older when removed and still worries and feels responsibility towards her. She also tends to fantasise about her, and letters expose her to the reality that she can't live with her mum. They don't have to worry about whether or not she is doing well, they don't have to wonder when they have questions. DD2 knows through writing that her mum also has talent for sewing, knitting so that's where she gets it from! She knows who she looks like. Both DD and DS like hearing from their brothers and sisters.
However, whilst our contact is reliable, for many people letters can be sporadic. It's not easy writing for the original family usually, and often they don't feel able to do it, or don't know what to write etc. It's a tough call whether to share a letter with a younger child in case their hopes are raised too much and then another letter does not arrive. It's sometimes really hard for the adoptive parents to write as well, no matter how positive the contact is.
Hopefully, the letters should be vetted (going both ways) so nothing totally inappropriate is said. That the contact is controlled and anonymous (depending on the situation) should satisfy worries about the fact they are drug addicts etc. You have to talk to DC about these issues anyway, as you start explaining why they were removed. Also, letters are generally written to the parents, and it's up to you when or if to show them to a young child. Many people put them in special boxes and wait till the children ask to see them
I think I've rambled a bit! Basically, I think it can be very positive depending on the circumstances. Everyone you ask will have a different opinion though