This very topic was the topic of a previous research paper that I wrote and consequently turned into a presentation.  I would be happy to email it to you if you provide your email address in a PM.
As far as your question, here are my thoughts:
Patient assessment is actually much easier following appropriate analgesia (this is supported by mounds of research as well as anecdotally).  What was previously diffuse abdominal pain with a patient writhing is not tenderness concentrated to a very defined area.
Effective analgesia is one of the few times we can make an actual difference in someone's life.  Pain is not just a nuisance, it is a significant problem which must be treated. 
Eric
P.S. Short of posting the entire paper in this forum, here are my references:
[1] Position Paper: Prehospital Pain Management.  National Association of EMS Physicians. Prehospital Emergency Care.  October-December 2003.  482-488.
[2] Why Don’t We Do a Better Job of Treating Pain?.  Bryan Bledsoe, DO.  
http://www.bryanbledsoe.com/pdf/handouts/PowerPoint/Pain Management.ppt#256,1,Why Don’t We Do a Better Job of Treating Pain?
[3] Pain Management in the Prehospital Environment.  McManus MD, Sallee MD.  Emergency Medical Clinics of North America.  2005.  415-431.
[4] Prehospital Pain Management: Current Status and Future Direction.  Hennes MD, Kim MD.  Clinical Pediatric Emergency Medicine.  2006. 7:25-30.
[5] Pain Medication Administration in Pediatric Trauma Patients with Long Bone Fractures Before Emergency Department Arrival.  Mader MD, Letourneau MD.  Annals of Emergency Medicine. October 2004. 44.
[6] Inadequate Analgesia in Emergency Medicine.  Rupp MD, Delaney MD.  Annals of Emergency Medicine.  April 2004. 494-503.
[7] Refusal of Base Station Physicians to Authorize Narcotic Analgesia.  Gabbay MD, ****inson MD.  Prehospital Emergency Care.   July-September 2001.  293-295.
[8] Simplyifying Prehospital Analgesia.  Bledsoe DO, Braude MD, Dailey MD, Myers DO, Richards MD, Wesley MD.  Journal of Emergency Medical Services.  July 2005.  57-59.
[9] Changing Attitudes About Pain and Pain Control in Emergency Medicine.  Fosnocht MD, Swanson MD, Barton MD.  Emergency Medicine Clinics of North America.  2005.  297-306.
[10] The Epidemiology of Pain in the Prehospital Setting.  McLean MD, Maio MD, Domeier MD.  Prehospital Emergency Care. October-December 2002. 401-405.
[11] Few Emergency Medical Services Patients with Lower-extremity Fractures Receive Prehospital Analgesia.  McEachin BSN, McDermott EMT-P, Swor DO.  Prehospital Emergency Care.  October-December 2002.  406-410.