spontaneous update, for my opnion only. because if theres any hickup during the road map like security exploits, more bug reports than expected which is more time consuming, it will eventually lead to any possible delays which will no longer correspond to the road map itself.
i think spontaneous updates is a more of a comfort zones for the developers, where they release their update at their own pace instead of forcing themself stressing out in order to keep themself within the timeframe for the roadmap they created. IB proved that point already when they released their 4.0 Gold, they announced the gold release dates several months in advance
I was looking to purchase license of Xenforo but for my new forums, Calendar and few other missing features are crucial. As a waiting customer for few features, I would like to purchase the license 3 months before, so it gives me enough time to advertise my forum on my other forums.
Nothing should be perfect and released on same month as specified but road-map helps new customer in deciding what features are going to be added. I was just about to purchase new IPB license but now I know new version is going to have custom profile field and few other things which I was waiting for.
Roadmap is deciding factor at least for me because at present Xenforo doesn't have all features that other competitors are having. With roadmap new waiting customer at least will know what they will get in next 6 months and then they can decide either to wait or go ahead with other options.