Darius,
This is a bit of a large topic, and you will hear varying opinions on the subject. In a nutshell, there are advantages and disadvantages as with most issues in this business...
On the Plus Side:
1. You get some additional bass reinforcement below 500Hz (or so) from the extension of the speaker baffle (approaching an infinite baffle) when you properly wall-mount your monitors. This is effectively 'free' low end response....
2. You basically eliminate the speaker boundary interference issue with the front wall, as there is no more boundary.. the speaker baffle (front wall) is the boundary. Note, that there is still boundary interference from the floor, side walls and ceiling potentially depending on how effective your trapping is on some of these surfaces.
3. When implemented properly you can create great stereo imaging and you can control the exact focus point thus creating as wide of a 'sweet spot' as you like within reason of course.
4. The 'main monitors' will ALWAYS be in the same position!! This consistency is a big plus as you and your clients will become accustomed to the sound of your monitors in your room and it should be a 'constant'... (of course this assumes that your monitors are maintained, and drivers are not blown, and the your cross-overs and amps are NOT accessible to curious and fiddling little fingers... YES, that is the voice of experience recommending that you mount your cross-over and amps where nobody can touch them!).
5. When properly implemented you can have an awesome, large sounding and accurate monitor system with lots of low end and sweet imaging.
6. Clients are very impressed (assuming they sound good).
7. Some monitors are actually designed for this type of installation and won't sound right unless they are (properly) wall mounted.
On the Minus Side: (See all of the above)
1. You may not like the extra low frequency from your otherwise well-balanced monitors.
2 & 3. You are now STUCK with the position of your monitors, for better and for worse.
4. The monitors will always be in wrong position!!
5. When done wrong, your reasonably high quality monitors will sound like ass!
6. Clients will be impressed, until they leave your studio and say how God awful the main monitors sounded!!
7. Some monitors just sound better in the free-field.
Actually, for many installations we do very careful and detailed in-wall monitor installations. We try to design allowing for the inevitable book shelf monitors that will be stuck up on the console in the way of the mains.
In many other installations, we recommend the client stay with free-standing or free-field monitors. The advantages are:
1. The ability to change monitors at will!! (this is a biggy)
2. The ability to move the monitors around while you 'learn' your room and find the best monitor and listening positions.. (this is my biggest plus point). I can't tell how many times I get paid good money to go to somebody's new studio and move around the monitors until it sounds good!! This is the single biggest thing you can do to improve your monitoring situation in a small/mid sized control room with free-field monitors...
There is so much more to say about this subject, such as discussing how to actually do it correctly.. but I'm starting to get tired of talking... now, there's a topic.. talk amongst yourselves!! (just kidding)..