MMA is a fairly new sport, and as such most of the people training it have been training and even teaching for longer then it has been around. Due to this, old traditions are often held onto, despite being best suited for a different goal.
Most of us teaching came up learning the skills used in MMA through different styles, different instructors and seperated from each other. Often this leaves behind a sense of tradition, where instructors feel that because they learnt one way, there students should learn the same way.
However this leads to a lot of wasted effort on skills and tactics that do not apply to the current goal.
When the goal is MMA, MMA should be what a person is training in. Cross training definately has benefits, but you’re primary goal should be your primary training method. Other training should supplement, not form the basis of your primary goal.
Freestyle Wrestling may have some of the best, and most applicable takedowns around for MMA, but the ground work is completely different and many “good” habbits in wrestling are extremely bad habits in MMA where you generally want to face your opponent, not face the floor. Spending time focusing on using collar chokes and sleeve / lapel grips as setups in grappling will also not help when your opponent is not wearing a gi.
Thats not to say it has no benefit, the gi does force a slower pace and more technical escapes as a result of the extra grips and slower pace. But should it form the primary method of training ground work in a sport that doesn’t use it? Absolutely not.
MMA has evolved into a unique sport with its own techniques, tactics and drills. From as early as possible, Day 1 preferably, every situation should be looked at as it applies to MMA. Your striking should always take into account an opponent who clinches, pushes you up against something or tries to take you down. Your guard work should be based on a opponent who is trying to throw punches and is slippery and hard to grip.
There is however one very large advantage to training in a discipline seperate, and that is amateur competition. Right now pro-MMA is fairly well established, unfortunately there is much less for amateurs who often fight under largely the same rules as pros. Getting some experience in a more established amateur combat sport such as Submission Grappling, amateur boxing or amateur Kick-Boxing prior to going into MMA is definately a good plan for a aspiring fighter.
However I do think it is a mistake to try and spread yourself to thin. Each individual sport has a huge amount of depth on its own. If you are trying to be a competitive Grappler, Boxer, Kickboxer, Wrestler and MMA fighter, chances are you will not reach your full potential in any of them. There simply isn’t enough training hours in the day, and that is ignoring the fact that good habits in one, are often bad habits in another.