Air conditioning system should be vacuumed prior to opening the valve at the outdoor side.
Vacuuming will remove air and moisture from the piping and the indoor unit.
Unfortunately, very few installers (not to say none) will perform this proper procedure here in Malaysia (unless installed in projects such as new properties which come with air conditioners).
What they normally do is to use the refrigerant from the outdoor unit to "flush" (force) out the air from the piping and indoor unit.
This may cause:
1. Premature failure of the compressor (not immediate though)
2. Insufficient refrigerant to the system, which will cause a drop in cooling capacity
3. From no. 2, drop in energy efficiency ratio (EER) - paying more for electricity compared to the cooling you get
4. Abnormal noise

did you just answered your own question? 

i don't think it is due to the potential repeat business
this is what we get when we do not have proper bodies to control the industry
anybody can become an installer
mostly do not have any professional training, what they know is just what they learn from the people they are working with, doing only what is normal to them
can't exactly blame them as they are also paid lower if compared to Thailand or Singapore
as cheap as a vacuum pump can be, it is still a cost, and the vacuuming time = extra time spent per installation = loss of revenue

things will not change overnight
only with government initiatives to control and certify, and the willingness of us as consumers to pay a fair price for quality installation then only the situation can improve

Most customer not willing to pay more for extra procedures.

Common reply for 1hp as below:

RM200 - "can cheaper a bit or not, I buy 2 units leh"

RM250 - "your installation price is expensive than the other shop"

Rm280 - "1hp or 1.5hp charge?"

It is not common to see customer willing to pay higher for extra jobs.

Just thought I clarify your question. Usually if new aircon, installer don't vacuum cause new unit have extra bit gas inside to purge out when piping are connected. This extra gas are put in by the manufacturer straight out of factory.

You only do vaccuming when you moved the aircon to another location, reconnecting piping etc.