In addition to the other answers here ...
In my experience, having your active dev sandbox get upgraded to the pre-release version while you are trying to get day-to-day work done and fixes pushed to production can sometimes be exasperating as the pre-release sandboxes may have bugs wherein Apex code or other platform functionality stops working for days or weeks until repaired by the SFDC developers.
Just look through past SFSE posts like this one to see that upgrading can be an 'adventure'.
That said, SFDC does an admirable job of keeping existing code functioning as they go through the release introduction process three times/year.
Obviously, if you want to take advantage of new features you need to upgrade.