I've recently been working in an R&D group, developing various features that are quite experimental and wouldn't normally be scheduled as part of a project. This is intended to be "practical" R&D - we aim to develop features that have a chance of making it into the game, as opposed to features that are blue sky and would never actually be shipped.
This kind of development approach subtly shifts your approach to developing features. Briefly, you become very focused on doing what is necessary to get the feature into the game. You focus on making sure the feature is on-budget. You make sure that you consult with affected parties. You work out the consequences of incorporating the system and seek to mitigate any problems. You suddenly find yourself becoming a salesman, trying to establish a coalition of support for the feature. Your feature really has to prove itself worthwhile so that management will entertain the perceived risk of incorporating it.
Strangely, you don't always find yourself doing these things during the development of "standard" features. Because they are planned as part of the development effort, their inclusion in the title is automatically assured - pending no major disasters. You don't have to sell them. These features don't need to earn their place. Consequently, it can be very easy to find yourself "going down a rabbit hole" with features like this - exploring functionality that is interesting, but not necessarily focused on shipping. Perhaps these features are ultimately more risk-intensive, as they won't be quite so critically examined as the R&D features.
I tend to find that the whole "posture" you adopt during the development of an R&D feature that is not automatically included in the game is ultimately much better from the perspective of careful software development. It does at least feel more like the kind of process intended by the creators of various software development methodologies. It feels very much along the lines of the approach advocated by Michael Saladino in his "Why Red Lining Your Team Leads to Ineffectiveness".
Perhaps it's not for everyone. Perhaps it's not for every team. Perhaps some may find it quite a pressured, stressful kind of approach. But I really quite like it. It certainly feels a much more considered, deliberate approach. It seems that developing features in branches, that have to prove their worth for integration, sets in motion a slightly changed, more focused set of processes than may be typical.