Q: Why do post permalinks use the thread URL instead of the absolute post URL? Doesn't this mean that post permalinks won't work if i move posts or merge threads?
A: Originally, we did link to the absolute post URL, but in the spirit of optimising for the most common case, we changed it to use a link relative to the thread in which it resides. Why?
Firstly, in most cases, posts won't be moved.
Secondly, and more importantly, if you viewing the page threads/permalink-related-question.3417and a post contains a permalink to threads/permalink-related-question.3417/#post-51426, a search engine spidering your site will recognise that this is not a link to independent content, and will not follow that link. However, with posts/51426, the search engine can not know that, and you've wasted dozens of spider visits for every thread the search engine hits.
On balance, we felt that the second reason was far more important than the possibility of losing a post due to a merge or move.