Some good points have been made for tutors/teachers. I'll make another one - it saves me time. I want to talk for 1hr per day in my L2. A language exchange would require me to spend an extra hour speaking English. I'm old, and the time I have left in this world is very precious.

It seems the only real minus listed so far is the fact you have to pay for tutors. This is probably the biggest single factor for most people, and outweighs all others. And I do mean <em>most</em> people; there are probably at least 10 times as many members seeking partners here as taking lessons.

However, there's another potential minus that I'll discuss. Many people feel they can relax more, have a casual conversation, and really get comfortable using the language with a partner. This sometimes isn't possible with a tutor. Now I'm an exception; I use tutors for conversation only, and make sure my tutors know I want a casual conversation with as little correction as possible. However, I suspect most students are trying to accomplish more than just getting comfortable with the language when they meet their tutors, so they look to language partners to fill this need.

I don't see why a person can't do both.

I agree that it is so cool to have somebody to actually chat with, that will be your first connection to your   target language.

It can take ages to find the right person that will actually show up, so in the meantime, why not actually practice with a  community tutor ?

I have had some who were very relaxed and with whom I have a great relationship.

About language exchange partners being free, that is debatable. Time wasted has a much higher price than we realize ! And a lot of time is wasted in the process of finding the right partner that can also keep a conversation going.

There are cons and pros, of course. 

If you find the right language partner , you could learn any language pretty well with him/her.

When you try to teach your mother language to someone, you could understand in a different way the process of learning, so probably it'll help you a lot.

On the other hand if you don't want to spend time teaching, you should pay for a tutor. 

A tutor could guide you better during the learning process.

I use both.  I was lucky, probably because I am a native English speaking female, in that I have gotten quite a few requests to be a partner. The first three partners I have accepted all turned out to be good responsible partners and scheduling is not difficult- we each have pretty much a weekly standing appointment.  I prefer those partners to any teachers or tutors not because of the cost but because they do not see it as their job to make me speak my target language the whole session, they send me links to videos and articles they find that they think I might be interested and treat me in a friendlier, more casual way,  and I enjoy the process of helping them with their pronunciation and helping them learn new words and expressions. I am an independent learner and I do not like it when someone else attempts to direct my learning, which teachers or tutors sometimes do. I usually just do not return to those tutors or teachers who I feel are trying to take too much responsibility for my learning.  

I have not found a consistent partner for my morning times nor am I able to be consistently available on Saturdays.  I use paid teachers or community tutors to get more practice   I like to work on what I think are the more boring things, like verb conjugation, with my paid partners.  I especially like the instant tutoring option because often I want to practice at times when I could not plan ahead.  I just wish italki would make an option for instant tutoring for an hour instead of just one half hour at a time. I also do not feel like it is appropriate to be as informal with them.  I like to send my partners links to things in English I think they would find interesting or helpful.  I do not feel like I should do that with a community tutor.  The relationship is more convenient but feels a little more professional and a bit less friendly.