Pentecostalism is as about as a specific term as "christianity" or "monotheism" or "paganism" comes when it comes down to actual beliefs, practice of worship, doctrines, etc. The largest of those groups, "Assemblies of God" defer in key areas of doctrine, faith, and worship from the next 8 or so on that list. You may argue the differences aren't worth the seperation, but consider even in the American Protestant landscape -- even the Presbyterian Church, which is a specific kind of Protestant is heavily fractured between it's more liberal wing, the PC-USA, and it's more conservative group, the PCA. Baptists not apart of the Southern Baptist Convention differ heavily from those not a part of the SBC. Methodists and Lutherans face similar key distinctions. If all of these groups were united in belief, practice, doctrine, and key tenants of the faith, I would agree with your statement, however this is not the case.