The other option: { {{{a}}} } seems like it may have been what was intended in some cases. Especially if you define triples as being especially closely bound.
I think that's the most likely intention. I can't think of any time when you would want to call a template with the name coming from another template. I doesn't seem very unreasonable to have a parameter being displayed in curly brackets.
Well, not as obvious as a nice fat parse error would be. :)
Ah, now that's a good idea: If you don't know what someone means by something, ask them.