That's not clear to me at all. There's no inherent good in avoiding dependencies among Boost libraries. It's a judgement call, based on many factors, including the author's personal preference. You have every right to make that call yourself, but if you're trying to somehow *justify* your decision, IMO so far you haven't said anything that demonstrates it to be the best one.
The reason why ptree does not use multi index is because implementation existed long before I considered submitting to boost, probably before even I knew of multi index existence. It was working well. Later, when I was improving it during pre-review process, I seriously considered using multi-index. But I decided it is not worth throwing everything out. Although ptree has large interface with many functions modifying state of the tree, it uses "single point of change" approach. Every insert eventually goes through one function, which takes care of exception safety and keeping index in sync with data. The same applies to erase. This function has 9 lines of code in case of insert, and (by coincidence) also 9 in case of erase. By using multi index these functions would obviously be simplified, maybe to 4 lines each. Net gain: 10 lines of code (out of several hundred in ptree_implementation.hpp). I'm aware that there are performance gains to be reaped as well, but at that time I was rather focusing on getting the interface right. Best regards, Marcin