Someone who is not an attorney cannot provide legal advice. In addition, the aren’t bound by the same ethical obligations and they aren’t covered by malpractice insurance.
Someone who does not charge flat fees for the entire application process cannot provide a predictable “all-in” total cost for the process if there is a rejection or more information needed, or an Office Action from the USPTO.
Someone who was not an examiner working at the USPTO in the past hasn’t seen the process from the inside and cannot have the same depth of knowledge of how the USPTO evaluates applications.
I suggest that someone with all three qualifications — an attorney, who is a former examiner, and who charges flat inclusive fees – provides the most valuable help possible when it comes to filing a trademark application. But then again, since this describes me I must be biased!