i have a 17-55 dx nikkor. and i don't think i will sell it even if i go full frame. most likely will use it for a lower end body for travel usage

it's hard to compare it side by side with 3rd party lenses. you must use it then you'll feel it.

for me it is a lens that you learn to appreciate with time.

^ I only owned 35mm , 50mm 

To be frank, I have seen some wedding photo shots slightly OOF and all. It just boils to how much they care... sometimes as long they get the moment, slightly oof and all is ok. Some people demand more some people demand less. Up to the user and client, most of them can't differentiate between sharp and slightly OOF anyways.

i always set M , 800 / 1600 ISO , shutter speed 1/40~50 , TTL flashgun then good to go

i using canon 550D  , ISO 800 / 1600 isn't that bad la actually , shooting indoor normally at that ISO also , summore shooting group pic mostly using f4~5.6~7.1 , seldom use lower than f2.8

Depends, sometimes, you want the ambience/mood, sometimes, you want the skin color; especially if you're shooting people (especially girls); Imagine their face turn out yellow or orangy or dark when they have fair skin?

And the ISO is not really high especially if you're shooting wedding reception indoor where it's low light, and you're shooting group which requires smaller aperture most of the time.

If you underexpose, then it will be worst, the noise will be more apparent and muddy. So, if you underexpose, you will think or have the mindset that the camera cannot go higher ISO, when in fact, it's not the camera's fault.

ghost month is near, if you shoot orange/ yellow skin girl and got result in white skin... habis lah 

You obviously have not encountered problems with matching the flash WB with the ambient WB or p[robably do not realise that is a problem in the first place. If you do not correct the WB light from your flash to match the ambient lighting, you will get different colour lighting from the ambient and from your flash in your pics. Correcting WB on the subject in PP (as a result of light source from the flash) will produce horrible outcome on the background and vice versa.
If vearn27 wasn't using flash, doing custom WB would be the best. But if you use the default flash in tungsten ambient lighting, you would have problems getting correct WB in your pics if your flash is not WB corrected.
Like I mentioned in my reply above, even PP in RAW would give horrible "corrected" WB in PP if the flash WB is different from the ambient WB. Your method is correct if vearn27 does not shoot with flash.
Not possible if you use flash that is not WB corrected.