I would prefer that anybody teaching English, in any country and at any level, holds a relevant degree and a PGCE (as I do). However, a DELTA qualification is an exceptional qualification for any teacher of English to hold. (Note I am saying DELTA not CELTA.)

The DELTA is a professional qualification in English language teaching consisting of three modules awarded by Cambridge English Language Assessment,formerly UCLES, a part of the University of Cambridge. The diploma is often seen as a follow-up to the certificate known as the CELTA,once the individual has done at least two years of teaching and has decided on a more long-term and serious commitment to the teaching of English. The diploma is awarded upon passing three modules covering a range of skills and topics:

Module 1 is a written examination on language and methodology Module 2 involves supervised teaching practice, observation of other teachers, and completion of a range of written assignments Module 3 involves in-depth research and an extended written assignment

Although candidates are required to have experience and to demonstrate aptitude before beginning the course, a substantial minority of candidates (about 30%) fail module 1. For each of modules 2 and 3, less than 2% of candidates' performances are actually graded as a "fail" but about one sixth of candidates are given a "referral" and cannot pass unless they successfully reattempt the evaluation.

The Delta is available in many different countries throughout the world. The course can be taken part-time over a year or more, or full-time over a period of two to three months. Distance learning is also available. These intensive courses provide input for Modules 1 and 2 but Module 3 cannot be taken within this timeframe. It is widely recognized around the world, as is the CELTA. The Delta is accredited in England by the Qualifications and Curriculum Authority at Level 7 on the UK National Qualifications Framework as are master’s degrees in related subject areas.

I don't believe the problem with language mills is purely the business model, but the fact that in many Asian countries there are no regulations regarding who is teaching and how qualified they are. This in turn leads to people who are there only to fund an extended vacation and not really to teach, meaning unscrupulous bosses can get away with a lot more than they can with qualified teachers.

The same is not true in Europe, where qualifications and credentials are checked thoroughly prior to employment, including criminal background checks and extensive reference checking. If the same were true in Asia, standards of ESL teaching would skyrocket in the space of a few years.