JR - I think the greatest value of the IMC and the qualifications it offers is that combined they help independent consultants and small firms to overcome the credibility issue when trying to win business with major client organisations.
I've met with a panel of FTSE buyers of consulting services in the last year - and it was very clear that they viewed the qualification as something that allows them to step outside their comfort zone (of engaging the major consulting brands, "no one ever got fired for hiring IBM" mentality)... and to consider engaging small consulting firms instead. The qualification is therefore seen as a stamp of approval of the quality of an unknown / untested consultant - from the purchasers' point of view. So potentially worth investing in if you are going down this small firm / independent route yourself.
I would almost go so far as to say that the qualification has a higher prestige amongst buyers of consulting services than it does amongst the consulting industry though...
... of course, strategically, the major consulting brands have a vested interest in not giving the qualification more than a token level of support. After all, they've invested a fortune in building up their brands as a means of reassuring clients that they are being wise in using them as their consulting supplier. Strengthening the reputation and reach of the IMC qualification is therefore helping small upstart firms to get themselves established. Not exactly good news for Partners at major brand consultancies! And hence not something you see extensively pursued by the majority of big consulting firms.
I would expect that this thread produces some disdain from consultants at major brand firms - for whom it is not at all essential to their everyday work; but that independents and small firms will attest to the value of the endorsement that the qualification gives to their work.
So whether it's useful for you to have really depends on which of these camps you fall into. Hope that's helpful.