The TEFL certificate is not currently a legal requirement in Thailand. However, many employers still want to see one. If two teachers go for a job interview; both have degrees and one has the TEFL certificate as well - guess who the job will probably go to. Having a TEFL certificate can do you no harm at all.
Having a TEFL cert will make you more employable to majority of employers. True it's not necessary but having one will open more doors for you.