Let's consider for example the following graph templates:
The template is referencing two string properties. When using RRDtool, the title is rendered properly. I mean, if the value of the variable is null (or the variable doesn't exist on the resource), it is rendered as a empty string (as expected).
When using Backshift, instead of an empty string, you'll see the name of the string property, which can confuse users (see backshift-issue-with-string-properties.png, the resource doesn't have a value for rzdAPDescripion).