Having been primarily a backend developer for most of my career, I generally view these services with suspicion. But the rate of new entries into this space makes me wonder if one day I'll be considered a luddite for thinking this way.
I have had the same reaction. I am not sure what customer pain these services are addressing. Even if it made prototyping and MVP developing vastly easier, what happens if the product starts to take off? The lock-in seems dramatic, but I could be off-base.
I believe the customer pain is that for many applications, it takes many iterations before the product takes off -- and for non-developers, who have to hire developers to implement each iteration, it could be cost-prohibitive to get to the take off stage. Easy application prototyping and MVP developing products allows agility in fine tuning ideas to make sure it meets a need, before spending the money to hire a developer and implementing the final, more optimized version.
It's also advantageous for developers to use these products as well -- not only is it faster to develop (no need to learn a ton of APIs), it's also easier mentally to change the product to suit a business need (after all, it's easier to toss out an afternoon's worth of work than 2 weeks', if needed)