The LiMo foundation has released an economic analysis that (among other things) explains business and economic reasons for 'deforking', i.e. contributing vendor-specific changes back into the upstream projects.
If you don't want to read the full paper, skip to Chapter 4.3 (Page 20) where they say things like: It is important that mobile industry platform providers engage with the open source communities as early as possible so that platform maintenance strategy is fully aligned with the upstream development agenda of these communities, which is far more cost efficient than managing the entire maintenance burden in-house.