[Serusers] Loose Routing ramifications

Hello list, I am new to SER and trying to understand the ramifications and downsides of SIP Loose Routing versus Strict. I have read RFC3261 and some net documentation, but I'm still having trouble seeing the big picture. In particular, we are testing a SIP route to an ITSP, and they have specified to us "no SIP loose routing". I'm trying to understand, why do providers do this? What is the downside? What are Industry Best Practices? We have some downstream customers that might be connecting Asterisk IAX to us, and then we will be routing these calls via SIP to the ITSP provider. It seems to me that there might be ramifications for loose versus strict routing in how we process these calls. Can anyone with SIP routing experience explain best practices or Advantages/Disadvantages, or point me to a link? I found a document that explains the 'problem' with Strict routing: “Strict” routing (Route/Record-Route as defined in bis-05 and before) was too strict. Service logic could not affect routing of the initial request. * Strict routing conflates the request target with the next hop destination. * Strict route processing throws away the information in the received Request-URI. * Behavior of UAs with default-outbound-proxies problematic. * Brittle system failure if any element misroutes. TIA, Jason Ostrom