bit confused by that last bit, as you say go train in boxing but boxers were getting beat a lot in the early UFC days
which other systems are you talking about btw? as a lot of the top mma fighters all have trained in other fighting techniques as a base for their skills.
most other systems do have their own competitions but theyre just not as big as ufc/pride(was)/strikeforce/WEC etc.
Boxers got their ass kicked in the cage by wrestlers and ju jitsu people, I have never, in all my days of witnessing fights in town, seen anyone go for a double leg or ever even need to think about sprawling in a street fight. It is all standing and banging which is where boxing is at its best.
Useless training systems: Aikido, Wing Chun, Karate (the way Karate is now compared to its kick boxing roots), Wushu, pretty much any Kung Fu, Ninjutsu, Capoeira, Taekwondo
The problem with these "Martial Arts" having their own competition is rules, a real fight contains no rules. In a Taekwondo tournement your not allowed to punch to the face, so we silly antler style hop skotch kicking while leaning back which would get you flattened in a street fight.