Nope...

30 days can be extended by 30 days. And if you need more time, you can go pay again, and they will then give you an extra 7 days to get the fuck out of their country.

But how is it useful in Thailand?

Worthless advice...

You can stay 90 days. 60 days plus 30 days extension.

I tried the same in Phnom Penh, VOID with no explanation.

I still got in on my visa exempt and haven't had any problems since.

Visa excempt, 30 days. Extendable one time for another 30 days.

Fly out to Vietnam, Malaysia or do a border bounce to Laos and get a SETV, which will be good for 60 days, plus 30 days.

60 days on tourist visa, add another 30 days at immigration for 1.900 THB and add another 7 days at immigration, again for 1.900 THB. Then you won't have problems visiting the country again some day.

Or you could visit a neighbouring country while you are here. Return flight to HCMC is usually around 2-3.000 THB and then you get to see something else as well.

Yes

It's called "MULTIPLE ENTRY" for a reason :)