The reason why it makes sense to use an entirely different language for the templates is quite simply to avoid using Perl
If I may add to this... In one of my past projects, we used templates "of our own" to present data in tables and graphs. Because of short deadlines and heavy workload, we eventually had to fix some bugs here or there and because of the same-language commodity (as well as a few other reasons), we started mixing together the template code and the logics... ...which naturally impacted the development of future modules forcing me to save HTML tags with the data inside the database (yeah I know! that's a horrible thing that still gives me nightmares but I'll get over it eventually).

So I think it's really worth separating data and logic. And language isolation is also a good thing since it protects you from yourself.