Templates should be about how things look like. You'll need some minor capabilities to style things dependent on values, but it's a slippery slope: You start out with branch on "exists", then later see that you need to style every even row in a table. Next project you need to style every third row and decide "oh, I can generalize that". And then you end up with a full math implementation. And wouldn't it be awesome if we could branch depending on the length of a value? You start hacking and the next thing you know when you wake up in the morning is that your template language is Turing complete. Now you have another language to learn.

It's far easier to make a cut at "no logic apart from boolean checks" and then use a proper client-side framework such as backbone if you need full view logic on the client. It separates concerns and makes clear where logic is supposed to reside. I love separation of concerns!