Proof of this is found in this book. In many examples you'll find the use of inline scripts or event handler attributes. It makes explanations and demonstrations easier while at the same time making maintenance a headache. The more tightly you integrate multiple layers, the more work you face when making changes down the road.
Using inline scripts and event handler attributes is technically correct. After all, the examples in this book use them, but their use made the examples harder to maintain and, in the case of errors, fix. Consider the following code as an example:
There is an error in this code. But where? Is it in the
onsubmit event handler: ...