Most (industrial) abseilers that I've come across are sport abseilers who have done the IRATA course, work for a while to build up the pennies for a long abseiling holiday, then come back to do the same again. With the exception of myself and a few colleagues who were "roped" into it (now all long past it), I've only met the odd bloke who had a trade or relevant experience before becoming a dope on a rope.
The prevalence of buildings that have been poorly designed (i.e. lack of adequate access to maintain the facade) or cheapskating on traditional access methods has allowed the cowboys a foothold in the building construction, maintenance and demolition industries. Over a period of time, abseilers have become experienced in what they do, but lack of training and supervision mean that they may not be particularly skilled. (You don't have to be skilled or knowledgable to be experienced, you merely have to have been doing it for a long time).
Having said that, some have learned from their mistakes and now have an inkling of what not to do, but truly good firms and individuals are few and far between. Look around and you'll see that many abseiling firms place the emphasis on gaining access to difficult places, rather than having the skills necessary once they get there.