I noticed authority’s in Thailand makes more and more crackdowns and spot-checks on foreigner in order to make sure their permit to stay is in order and work permits are valid if any.
As a person leaving your home country you are a foreigner and your only valid form of identification is your passport, of course there are always exception to the rule, for example; being a Schengen member I can travel within Schengen on a national ID-card.
Now what ever people will tell you, a Thai drivers license is not a valid ID for a foreigner in Thailand, it is not even a valid ID for a Thai citizen. You will here people claim it is good enough because your thai license has your passport number on it and authorities can check it, well I have a bike license and a car license one of two will always have an old passport number because renewing a passport does not mean one can renew a drivers license that is renewed every five years, it is possible to get a copy but expiry date remains the same.
Some people will say it is enough with a photocopy of picture page and the page with entry stamps, visa or work permit.
Well, a drivers license may get one checked in to a hotel or it may not, it may get one checked on to a domestic flight or it may not.
A few years ago I realized halfway to Chiangmai that my iPad bag with passport was still in Pattaya, well no problem my thai drivers license will be good enough. Arriving at a four star hotel in Chiangmai I was denied check-in then presenting my drivers license, because of the simple fact that it is not a valid ID, not even for a Thai and as the management pointed out that it does not necessarily have the same passport number as my current passport and what if they reported an expired passport to the immigration, a passport that I did not enter the country with.
A friend got denied boarding a domestic flight for the same reasons, this was a few years ago.
Personally I always carry my passport then I leave our home in Banglamung, around home I carry my Thai drivers license photos of passport pages in my phone, this may or may not save hours in the monkey house while they verify my identity, it may even let me off the hook without fuss. But hey! To be sure do not travel around in Thailand with your passport safe at home, its your only proof of identification.