I like the free tour for a number of reasons. First of all, you get to meet a local; someone who is always very passionate about their city and their country; happy to share many interesting facts and stories about its people, buildings and culture. I have learnt some fascinating things from these free tours. The quality of the guides varies. The tours where I have laughed heartily, learnt lots about the place and felt like the guide was genuinely interested in our experience have been fantastic. The tours where the guide has been very serious or a little eccentric, making the tour all about themselves, I have found less interesting and have tipped accordingly. Secondly, free tours are really useful in helping you get your bearings; allowing you to see the highlights of a city in a short space of time, but then give you the opportunity to decide what you want to visit again and see in more detail.
Free tours are a great way to meet other like-minded travellers. Free tourers are usually backpackers, probably travelling solo (and on a budget) just like your good self. On the tour in Ljubljana, I was able to meet a great girl who me and a fellow traveller, from my room in the hostel, spent the day exploring the city with and dined with in the evening. Further than that, the people on the free tour might become future travel buddies...an Aussie girl I befriended on the free tour in Riga I am travelling to Krakow with in October.
So, take the free tour...learn about the place you are visiting, soak up the culture and meet people!