In fact, for the majority of WordPress business sites, the performance gain alone would likely make financial sense - after all, how many more conversions would your site make if it loaded a second or two faster? (think customer satisfaction, SEO benefits and so on).
In a nutshell, using managed WordPress hosting gives you not only a precisely tailored environment in which your site will grow and endure traffic surges, but it also gives you peace of mind in terms of additional stability and security.
While managed WordPress hosting is pricier than shared hosting for the vast majority of sites, the benefits are worth the increased cost.