The move from 5.1 to 5.2 was a big move. Large changes in the renderer, a totally new JavaScript engine, and a lot of other usual changes.

It took a team of people (let's say, ~2-4 people) some months (let's say six) of work before it was ready for the rest of the company to take a look at it, and unfortunately some rough seas elsewhere in the company meant that it took even longer after that to mature properly (another 3-4 months).

In my opinion, the move to subsequent versions is less drastic, although there is still a (constant) influx of bugs and performance regressions to fight against -- and this together with the tablet development means that there likely isn't going to be a lot of priority to this type of work unless it brings substantial benefits in another area, much as I'd like to see it happen.

When it does happen, I imagine that it is likely to happen a lot more piece-by-piece: QtMultimedia is already being upgraded in a future release because it's needed for some other bits and pieces, for instance.

And to answer the initial question: an LTS release is a very good thing from Sailfish's perspective. It means there's a reasonably solid base that can be focused on (and collaborated on with other people), rather than a constant train of moving targets where Sailfish would either be on the bleeding edge or constantly far behind.