The reason is branding and SEO.
e.g. a web site with topics on sports on a separate domain / subdomain than a web site talking about cooking.
e.g. Someone sees sports logo and unique coloring on the iTunes listing, when they click to visit the web site they see something entirely different.
Those are the only issues. It is more a “best practices” vs “what is and what is not possible”.
Not to blow your mind, but you can also setup custom post types in WordPress (similar to content types and taxonomies in Drupal). Though rarely done, it is possible to use Custom Post Types and a customized theme that would separate each program and provide unique branding for each show while still maintaining WordPress on one domain. I would not go that route simply because you are then limited to only plugins that are compatible with custom post types, but still an option none the less. PowerPress supports post types, as well as taxonomy podcasting. The only time I advocate for post type podcasting or taxonomy podcasting is when you are developing your own theme and/or custom plugin to manage the post types and/or taxonomies. e.g. a sporting good store may use post types to organize products and each department have it’s own podcast to demo new products once a week, it would tie in custom theme development to display product information as well as silo the products into separate post types tied to custom taxonomies for navigating to specific products with breadcrumbs. PowerPress would then allow you to add podcast feeds to each post type, or even by taxonomy.
