Help with XML anti pattern

Hi XML folks,

I want to share here a real case to ask for your wisdom and advice:

This week my company started to work in a project involving a customization of UBL Invoices. A team before us, defined a structure similar to this for receiving the name of a person and the name of a software product:

<ext:UBLExtension>
  <ext:ExtensionContent>
   <SWMaker>
    <SWMakerInfo>
      <Name>FirstName</Name>
      <Value>Erick</Value>
      <Name>LastName</Name>
      <Value>Rich</Value>
      <Name>SWName</Name>
      <Value>FancySoft v.1.0</Value>
    </SWMakerInfo>
  </SWMaker>
 </ext:ExtensionContent>
</ext:UBLExtension>

This way of modeling data is opposite to the essence of XML, but frequently used by programmers that sells it as "flexible" because it can accommodate future uses with new combinations for elements "Name" and "Value" (and because many programming languages translate object dictionaries to this kind of XML).

The problem arrives when XPath expressions become more complex (although not impossible to write) to access data:

For example, to get the first name:

ext:UBLExtension/*:ExtensionContent/SWMaker/SWMakerInfo/Name[ . = "FirstName" ]/following-sibling::Value[1]

But this kind of expression is not trustworthy. For example, if the first <Value> element is missing, the LastName will be returned instead.

I'm asking for your help, because we'll have a meeting with the contractor and I need arguments to convince them of changing to a
"Natural" XML structure like this:

<SWMaker>
  <SWMakerInfo>
    <FirstName>Erick</FirstName>
    <LastName>Rich</LastName>
    <SWName>FancySoft v.1.0</SWName>
  </SWMakerInfo>
</SWMaker>

I'm expecting customer won't be open to accept the change, because the actual structure can do the work.

So, what arguments would you use to convince the customer?

Thanks in advance for all your input. It is very enriching to read your opinions and learn from all your experience.