There is a huge amount of value in constructive criticism, and really very little value in praise. In order to reach your full potential you must be able to handle criticism of your work. If you simply praise a person you are really not doing them any favors, in fact you might be hurting their development by creating a false sense of superiority that causes them to stop pushing themselves as hard to succeed. However, you really are doing a person a favor when you take the time to read through their work and offer constructive criticism, because then you are helping them to achieve their full potential.
In fact, it's a sign of respect to give constructive criticism because the implied message is: I found your work interesting enough to spend time looking over it and thinking about; and I'm not going to treat you like a child who needs to be coddled, but rather like a peer who is capable of handling criticism.
I don't mean value in the financial sense, but rather in the sense of helping a person improve as a developer.
As for mixing praise and criticism, that is appropriate for certain situations; for example when dealing with a student with low self confidence. However there is a danger in praise which I outlined in my previous comment, it really doesn't accomplish much besides stroking a person's ego, and that can have unintended negative consequences for their long term development.
In fact, it's a sign of respect to give constructive criticism because the implied message is: I found your work interesting enough to spend time looking over it and thinking about; and I'm not going to treat you like a child who needs to be coddled, but rather like a peer who is capable of handling criticism.