What's so bad about building XML with string concatenation?

Question

In the thread What’s your favorite “programmer ignorance” pet peeve?, the following answer appears, with a large amount of upvotes:

Programmers who build XML u

Answer 1

Another point against using string concatenation is that the hierarchical structure of the data is not clear when reading the code. In @Sander's example of Linq-to-XML for example, it's clear to what parent element the "product" element belongs, to what element the "title" attribute applies, etc.