Using additional weight is definitely needed for increasing the muscle mass, but it is also essential for more strength and endurance. MMA fighters do not need bulked up effect, but they do need their muscles to be able to withstand a long and strenuous fight. Those muscles must also enable the body to move as fast as possible. This is why MMA weight training routines are based on using basic movements, which are pulling, pushing, bending, twisting etc. This means that whatever type of workout is done, it has to include these movements. Why is that so important? Well, martial art practitioners have to learn and repeat constantly certain moves and techniques and that takes time. A balance between a good technique and strength is required, unless there is enough time to practice both intensively, which is usually not the case. Also, exhaustion might happen and that has to be avoided, especially when tournament closes by. Weight has to be set properly in order to have the best possible effect for a MMA fighter, and that is definitely strength and endurance.
What would be a good example of MMA weight training routine? As said, one day can be used for pushing, so all variations of bench press are allowed. Weight used and number of sets and reps must depend on the needs of the practitioner, but some moderate way might be the best, numbers that will not allow the muscle mass to increase drastically because that will affect the overall speed of the body. Second day would be for pulling, so pull up bar should be used as much as possible.
The problem with physical condition of MMA and any other martial art practitioners is that perfect condition is required. A body needs enough strength, endurance, stamina, muscle mass, flexibility etc. These all have to be in a perfect harmony for a practitioner to feel ready for a competition. This is why there is almost always preparing period that starts about two weeks minimally before the tournament, and very often it demands a quarantine.