How well defined is ADD/ADHD? I understand ADHD is a standard term, but I'm still a bit suspicious that it's not a useful one; Psychiatry doesn't seem like the most reliable field.

Are there good reasons for picking out the behaviors associated with ADHD and giving them a name?

Potential answers


Behaviors associated with ADHD strongly cluster
Analyzing
questionnaires of attention and focus behaviors with factor analysis
naturally produces an 'ADHD dimension' that explains a lot of
variance (similar methodology to identifying Big 5 personality
traits).
ADHD diagnosis is a strong independent predictor of something
interesting: income or grades or some contrived but interesting lab
test (controlling for obvious things like IQ) 
Something else along these lines.
Like all psychiatric disorers, ADD and ADHD are diagnosed using a set of criteria listed in the Diagnostic and Statistical Manual of Mental Disorders, or DSM

The latest version is the DSM-IV-TR. The DSM-V is due out in 2013 and may change these criteria.

Diagnosis is expected to be done by a licensed professional who is able to assess these criteria.

From http://www.mental-health-today.com/add/dsm.htm :

INATTENTION
(need 6 of 9)

- often fails to give close attention to details or makes careless mistakes in schoolwork, work or other activities
- often has difficulty sustaining attention in tasks or play activities
- often does not seem to listen when spoken to directly
- often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (no if oppositional behavior or doesn’t understand instructions)
- often has difficulty organizing tasks and activities
- often avoids, dislikes, or is reluctant to engage in tasks or activities that require sustained mental effort (such as schoolwork or homework)
- often loses things necessary for tasks or activities (e.g., toys, school assignments, pencils, books, or tools)
- often easily distracted by extraneous stimuli
- often forgetful in daily activities

HYPERACTIVITY-IMPULSIVITY  
(need 6 of 9)

- often fidgets with hands or feet or squirms in seat
- often leaves seat in classroom or in other situations in which remaining seated is expected
- often runs about or climbs excessively in situations in which it is inappropriate (in adolescents or adults, may be limited to subjective feelings of restlessness)
- often has difficulty playing or engaging in leisure activities quietly
- is often "on the go" or often acts as if "driven by a motor"
- often talks excessively
- often blurts out answers before questions have been completed
- often has difficulty awaiting turn
- often interrupts or intrudes on others (e.g., butts into conversations or games)

REQUIREMENTS

- Present at least 6 months, maladaptive and inconsistent with development level
- Some symptoms that caused impairment were present before age 7
- Some impairment from the symptoms is present in two or more settings (e.g., at school {or work} and at home)
- There must be clear evidence of clinically significant impairment in social, academic or occupational functioning