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

As you said, it's just awkward to build XML correct using string concatenation, especially now you have XML linq that allows for simple construction of an XML graph and will get namespaces, etc correct.

Obviously context and how it is being used matters, such as in the logging example string.Format can be perfectly acceptable.

But too often people ignore these alternatives when working with complex XML graphs and just use a StringBuilder.