Furthermore, in some cases I think it is a great idea.

I've seen to many "interesting" technlogies being crammed into projects just because devs needed them on the CV.

If people can test out everything they want without having to claim that they need it on a project it can save ourselves a lot more than 15% on any project or product that lives for more than a year or two.

At work we are still sometimes suffering because of how cool Redux once was.

To be the devil's advocate, you wouldn't want to pay your plumber for time on his pet project while he's working on your bathroom.

No, but I might prefer a plumber from a company that pays their plumbers to train and work on the latest 'tech' in their down time. I might even be willing to pay a 15% premium, especially if I'm looking for a 'cutting edge' solution for my bathroom.

You really don’t want a "cutting edge" solution for your bathroom. You want a boring one that you will be able to maintain by yourself for years with standard tools from the store.

And I truly think it’s the same for your codebase.

Given the choice, I would absolutely go for the plumber that spends some of their time practising difficult plumbing jobs and doesn't just do routine cases. Even if they are 15 % more expensive. I honestly believe they'll be much better equipped to handle anomalies, should they occur.

Looking at it further, I would ignore a 15% difference if it came with any enticing reasonable. Any small difference in quality is certain to return me much more than 15%.

this is such a poor analogy

I give you that, haven't spent much time on coming up with it.

I would like a competent plumber and if he does some fancy work in his spare time and gets even better than great.

But I would raise an eyebrow if beside my work he will also want to be paid for 6 hours he spent in the weekend, experimenting on his bathroom...

I wouldn't expect to have professional development overtly included in a short term contract, either. I'd expect a much higher hourly rate to pay for it though, amongst other things.

I'd expect if the plumber worked for a plumbing company they'd probably invest some time and money into skills growth, and pay for it out of the money they charge you.