This is posted by Lyn McKee (my login is still pending so am posting under Lindsey)
I believe that businesses still use the "percentage-of-sales method" in creating marketing communications budget for that exact reason because it is easier and that's is the way they have always done it. Humans are creatures of habit and they either don't know how to use the "objective-and-task method" or they don't want to put that much effort into it. By using the percentage method according to Kotler & Keller (2011, p. 499) "communication expenditures will vary with what the company can afford." They also talk about "encouraging stability when competing firms spend approximately the same percentage on their sales communications." This is a much simpler way to come up with a communications budget, the problem is the accuracy will not be there.
As for the "objective-and-task method" this makes marketing managers much more accountable in developing their budget. As Kotler & Keller (2011, p. 489) discusses, to develop such a budget a manager must look at several components. These include: "establish the market share goal, determine the percentage of the market that should be reached by advertising, determine the percentage of aware prospects that should be persuaded to try the brand, determine the number of advertising impressions per 1 percent trial rate, and determine the necessary advertising budget on the basis of the average cost of buying a gross rating point." As you can see, the second method is much more detailed and would require more extensive research analysis and data before being able to establish a proposed budget.