I have a couple variations of this style barrel, one of them an SVI. I'll give you my 0.02
1- I don't think it adds any weight to the front of the gun, it does reduce reciprocating mass of the slide which tends to reduce felt recoil.
2- Its not that it isn't useful on any other style gun (like an EDC gun) but it isn't cheap technology. Most people want something less damaging to the pocketbook in he event of losing it. There may be some draw backs to function in an environment where the action could get fouled, dirt or funk buildup around the island would prevent it from fully going into battery, potentially making the gun useless. I like the concept and Loe the way it functions in competition but I wouldn't carry a gun like that every day. Size, weight, reliability, expense etc are all factors.
3- It is frequently used on 9mm guns. SVI, Atlas, and Limcat will make them in 9mm and in .45. Im not sure why you think it wouldn't be considered.