Some critics contend that the use of the percentage-of-sales budgeting technique is illogical?

Sales is a function of advertising. Advertising should not be a function of sales. If a business has a bad quarter and the sales are lower, then according to this technique the amount of advertising/promo budget should drop as well. However, in reality the company might in fact need more advertising to raise brand awareness (or whatever the advertising goals are) to get the company out of the slump. The concept applies the other way as well. Studies have shown after a certain amount of advertising released the returns begin to diminish and have negative affects, even if the advertising campaign was successful initially. Therefore, a company should use an Objective and Task Approach (ideal method) or if need be a Competitive Parity Approach (match the competition).