The advantage of having a dictionary is expansibility: an implementation can accept or ignore additional information without polluting the namespace with additional accessors.  The advantage of putting *everything* in a dictionary is uniformity for the user: less API surface.  You don't need a special constructor and accessors.  

In addition, since users will normally construct request objects but not access their fields, a dictionary is easier to construct.  If it's just an alist, quasiquote; if it's a hash table, the hash-table procedure (which accepts alternating keys and values), etc.  Record constructors in Scheme are by order of arguments (aka "boa constructors"), which is hard to handle with more than a few components.  So then we get to pointlessly mutable objects, and people reusing them badly, and general messiness.