Why 138 combat is good?
Here is the problem, it shows your max combat in one third of the combat system. Say your maxed combat at level 200 with 99 defence and 99 ranged. Someone comes along with 200 combat and 99 range/mage/att/strength. The level 200 with 99 in all styles has much higher effectiveness since he or she has maxed in all combat skills so can effectively cycle through them to hit the one with just 99 defence and range with their weakness.
This is a big flaw in the system and the reason why max combat potential cannot be measured in just 1 style. Anyone with maxed in multiple styles is automatically more effective than the person maxed in just runescape gold. This doesn't just count for PvP either. Imagine in PvM, a higher level monster that is weak to a specific style. At maxed 200 combat you should be able to get best exp rates and kill it fastest since your maxed right? Wrong as you could only be maxed in 1 style therefore making you less effective if it's weakness is not that 1 style you are maxed in.
The ironic thing is that with EoC, Jagex clearly made it a goal for the combat triangle to be more a part of the game therefore effectively making it even less effective to be maxed in just 1 style then it was pre-EoC.
With the level 138 system it gives a better gauge of a higher range of stats therefore better reflecting your effectiveness in combat. The level 138 system has 1 flaw and that's the fact that if your melee stats are high, your mage/range don't effect your combat level. Jagex wants to poll the 138 combat with refinement ideas and even mentioned possibly making range/mage more of a factor in the 138 formula so problem fixed.