The problem is not only or perhaps even primarily with companies, but with non profit organizations, and pressure groups of all sorts. The problem is not primarily with skilled PR professionals, but less skilled people assigned to it, or simply amateurs. There's a good deal of COI editing of companies and products done by fans of the products, not by the companies, and some degree of conflict of interest permeates most of Wikipedia. Almost everything I ever worked on is either because I am interested in it, or want to show off my virtuosity. 
And I agree with Dennis that in most cases we might as well deal with COI on the article page directly, as with other edits. King4057, and others, have asked me at times to approve article drafts they have COI with I have refused to do so, because I will not take the responsibility for something I have not written, beyond seeing it is not blatantly objectionable. Let COI editors declare the affiliations, and then their edits will be judged appropriately. The only time that an edit needs approval on a talk p. is when it is dealing with something that is obviously susceptible, such as removing inconvenient facts.   
The true question is the same with this approach as with prohibiting direct COI editing:  how do we deal with COI editing from people who do not declare it. I see only two approaches. One is to have an high index of suspicion for every edit that might be influenced by conflict of interest, in essence abandoning the principle of Assuming Good Faith. The difficulty here is that very few can write material that is truly at a standard of sourcing that is proof against the possibility of bad faithit requires not only that statements be sourced, but being confident that all the sources have been examined and fairly considered and judge in context. Very little in enWP meets that standard. This would require actual research and fact checking at a degree which is beyond our capabilities, and amounting to what is done by the most careful editorial review. This is not work for amateurs.  It is easy to say, that if our standards are sufficiently high COI will not matter, but this  The other approach is to require all editors to be identified. The principle of permitting anonymous editing makes us vulnerable; it is necessary for those working on certain topics, but there it might have to be assured by some confidential mechanism for assigning names. Either approach means a radical change in Wikipedia.    
The only simple approach is the same we have used for other problems: patiently remove the worst of it, and go from there. '''''' ( talk )