I generally prefer JSON as a serialization format for a variety of reasons beyond what you cited:
1. Simplicity/plain text. Even a total newb could read a JSON file and probably puzzle out how the data works. This is important if I'm providing an export function or otherwise expect my users might want to shop somewhere else to deal with this data.
2. Forward compatibility/future proofing. If somebody finds my JSON file in twenty years with no documentation, (1) ensures that it's not going to be lost to them and they'll be able to extract the data with minimal difficulty.
3. Cross platform. It might not be the best format, but it's going to be supported in pretty much any language I would ever want to add support for the data to in a new system.
There's also a) it's fully schemaless and b) you can add new primitives. Which you may or may not need. But if you're looking for something that does a) and b) while being fast for browser communication, transit's currently your best option.
What does it even mean that something is string based?
Here: https://github.com/eishay/jvm-serializers/wiki a JVM bench where JSON is pretty competitive with binary formats [and results would be event better if project owners actually merged PRs ;( ]
Either way, the advantages of binary often outweigh the difficulty of having a human read the data. As the saying goes, code is executed far more often than it's read.
(Most binary formats have some fairly obvious possible text representations, at least for key fields (they're just encoding ints, floats, strings, bitfields, etc.). When you're having difficulty, or you're simply curious, you can print them out. If the format is any good, malformed messages are easy to check for in code - this code is not harder to debug than any other. If anything it's actually easier than with some kind of text format.)
I regularly MITM a connection between a mobile app and a HTTP server, when something isn't going quite right. A look at the JSON they exchange exposes the problem in under a minute more often than not. If I want to test something out quickly, I simply modify the incoming / outgoing JSON by hand. It's rare to get the syntax wrong and involves no context switches. I can then go back to the code, find the relevant section and make the changes I need to make. I find this a very convenient way of debugging and I don't think it would be as nice to do this with a binary format.