First thing, you should get the manhattan gmat sentence correction
bible. this is extremely helpful and will help you improve your sentence correction if you use the book wisely. second, you have to practice, practice, practice. look at the answer choices. see what words differentiate the answer choices. the most obvious errors on sentence correction are verb tense agreement, subject verb agreement, idioms, modifiers. please review these thoroughly and you will be able to at least eliminate some wrong answer choices.
the point you made about sentence correction
"I feel SCs are the least logical questions asked on GMAT. If MBA is more about logic and reasoning, shouldn’t they reduce number of SC questions on GMAT. Anyway the written English skills (grammar, sentence formation) are evaluated through AWAs and Univ essays."
i actually kinda disagree with you on this. i think sentence correction tests to see if you are a detail oriented person. this is actually an important skill in finance and accounting. i work as a hedge fund accountant and on a daily basis i have to catch mistakes that are off by 2 dollars on the balance sheet. so, as you can see, if you are a detail oriented individual it will help in this job. it's probably hard for you to see the relation between numbers and grammar errors, but i hope you can try to understand my analogy. on the other hand in gmat, i do not understand how circles, triangles will help us understand finance better but i guess it's just a way to test who knows the gmat better. ????
best of luck!
I'm very cheerful!