The cookbook suggests to use DataTransformer to strip_tags from input.

Isn't that violation of "Transforms a value between different representations"?

I mean, when u strip tags, it's not that representation of data changes – it's the data itself that gets changed. Stripping tags is a losing data procedure.

A cleaner way would be to use form event listener to change the data.

The example is almost right to use DataTransformer to change '<br>'s to "\n" and vice versa – almost, because the input could have both '<br>'s and "\n" – the original information, which should be where, would be lost as well.