Poor man's python templating - what should I worry about?

After churning through the various templating package, I now just use triple quoted strings and dictionary substitution. I never have to read docs, and never forget how to use it. Also, I never run into cases that break the system.

myhtml = '''

<html><body>

<p>%(firstname)s</p>

<p>%(lastname)s</p>

</body></html>

''' % {'firstname':'john','lastname':'doe'}

It's not pretty, but I feel less hindered by this approach. I'm curious what people think the main downsides of this are...