I am a teacher with said four-year degree and then two-year teacher training course, and I really, truly think the CELTA is an EXTREMELY valuable tool. Throughout my B.Ed program (after my B.A.) I would share knowledge and skills that I had gained during the CELTA, and my professors and partner teachers were always very impressed. The vast majority of the skills you develop during the CELTA will transfer to teaching in the K-12 or university system, in ANY subject (not just foreign/second languages). As well, the kind of feedback that I got during the CELTA was, in many ways, superior to the feedback I got during my B.Ed program because of the whole peer observation and feedback element, which can't really occur in B.Ed programs for a variety of reasons (mainly, scheduling and FOIP). I think that someone who takes the CELTA IS making "a concerted effort to be a decent teacher", particularly in relation to the expected payoff.
Six years of university allows me to teach in the public school system, and gets me $51,500 annually (in my first year, capping at $78,000 after about twelve years in my district), medical care, dental care, life insurance, a good pension plan, job security, very regular professional development, a mentoring program, my own classroom in one building, four and a half contact hours per day, a budget for materials and resources, etc.
Four years of university and my CELTA allowed me to go to Italy and work in a private language school where I was paid EUR 11.11 for my first ninety contact hours per month (22.5 per week) and EUR 10.85 for every additional hour of mandatory overtime that I worked, twelve hours per week of unpaid travel time to and from in-company classes, split shifts resulting in some thirteen-hour days, no benefits, no job security, minimal resources and minimal support. The payoff at private language schools simply isn't worth more of an investment than the CELTA.