I think you submit patches for share with other ppl... with community. Yes, when patch added to offcial sources it solve many problems for author: now mangos team problems have related code up-to-date, fix found bugs, and etc. BUT... ppl can use patch and without adding to official sources.
I for example (except cases when i write some big patch many days) don't know self what i will do next Dependent from last read threads or bug report or some point writed at some team discussion. I not very orginized man... I glad to read this. Maybe we all have different views how project must developed, but we all work at project development and improving.