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

I think readability, flexibility and scalability are important factors. Consider the following piece of Linq-to-Xml:

XDocument doc = new XDocument(new XDeclaration("1.0","UTF-8","yes"),
   new XElement("products", from p in collection
    select new XElement("product",
        new XAttribute("guid", p.ProductId), 
        new XAttribute("title", p.Title),
        new XAttribute("version", p.Version))));

Can you find a way to do it easier than this? I can output it to a browser, save it to a document, add attributes/elements in seconds and so on ... just by adding couple lines of code. I can do practically everything with it without much of effort.