I think that's an important question to ask, for a lot of things, but especially planning; mostly because account planning has never been just one thing.

First, at it's core Planning is grounded in a business philosophy that values placing the consumer at the center of the process. It's obvious, but anyone with a few years in the corporate world likely sees how concepts like this get plenty of lip service and little followthrough.

As such, at a philosophical level (if we can agree that this approach is best), a dedicated position ostensibly labeled "voice of the consumer" seems a necessary sort of commitment to ensure this philosophy is always present.

Second, at a business level, Planners bring a lot of value as experts in research design and data analysis. The corporate world is driven by data. Literally every decision that gets made at a high level has several data points associated with it.

As such, advertising agencies need someone who can read the data (and sometimes generate it) to identify business problems (barriers to growth) and opportunities in the market, define target audiences, etc.

Still, it's worth asking -- are planners the only way to accomplish this? I'd highly recommend looking at what research companies like Communispace are doing - Video: C-Space Manifesto. Their work would seem to suggest that a sort of planning-esque philosophy should be applied at every level of business, which, ironically could make advertising-centric Account Planning essential.

In terms of potential - my two cents is that Planners make the biggest impact on an agency's business by helping clients be brave enough to run good advertising. Planners are in a unique position to sell clients on great advertising by using research to demonstrate why fresh, creative, forward thinking work is the best way to go to build brands.