I've been having enough fun with this without buying the rest of the stages. Trying to collect all of the special challenge coins on the few three stages, doing the daily free run, getting tickets to do the challenge runs...that's enough for me.
I don't specifically feel a strong desire to lay out £8 for the other stages as I feel this is just a throwaway game, rather than an in-depth Mario title. I wouldn't pay money for Flappy Bird, and I don't want to pay money for this, either.
It's an odd opinion, even when thinking about it myself, but it's possibly because of how the whole mobile model normally works. I do wonder if Nintendo will eventually offer the chance to trade in points, coins, or set "win x number of challenges" to unlock more of the core game, or if it will ignore the recent backlash and stubbornly stick to its guns.