Scanning and skimming is NOT enough to get a high score on the general IELTS. If you need a 5.0, then it can work, but if you need anything higher (which almost all work and immigration purposes require), you need proper IELTS reading technique – you NEED to read and understand the material, and use critical thinking to engage the passage. This is the only way to get 6.5, 7.0, 7.5++.
Very high level ESL speakers may be able to achieve 6.5 using the scan/skim method, but they are still robbing themselves of a higher score. Such a student may be able to get 7.5 or even 8+ on the reading section using proper techniques. For most students, the difference between skim/scan and using proper technique can be a full band, or even 1.5 bands. It is absolutely huge.
There is a very good reason for this: The IELTS test makers are very intelligent, and they know exactly how to construct a test to differentiate between those using good techniques and those using bad techniques. The IELTS test makers design many questions specifically to catch students who employ skimming and scanning techniques. It is for this reason that these students find it very difficult to score above 5.0.
As with many things in life, if it seems to good to be true, it probably is. Skimming and scanning is an easy way to get a mediocre mark, but it is virtually useless if you want a good mark. If you want a good, great, or excellent mark, there is no better way than spending the time to learn proper technique.