Any of the ACE Inhibitors as mentioned are primarily designed to reduce BP as mentioned...they have the secondary function of reducing potential damage to kidneys through diabetic complications. You're not however being prescribed them for their primary purpose - reducing your BP even from a healthy amount, will reduce natural strain on your kidneys, so if yours have shown ANY sign of performing outwith normal parameters, whether that's the upper range of 'normal' or full blown failure, then surely you would want to do anything and everything possible to avoid serious complications from happening? You seem to take very good care of yourself in general, so I personally can't understand the reluctance to be on a medication that you've been prescribed to protect your kidneys, especially when you've been informed the results of a standard test showed some results worth protecting?
I'm now approaching stage 5 Kidney Failure - after years of not testing/bolusing correctly, and ignoring advice to take varying different pills like this it's now too late for me to stop what's coming. Mine have been prescribed by a Nephrologist, not a GP or a Diabetic specialist, but a doctor who has spent his entire career focusing on the treatment of kidneys and all their diseases/conditions. As much as I do not like the doctor personally (he's a smarmy git at the best of times) I trust his medical knowledge.
Take it from someone who is at the very far end of one of the potential paths you can travel here, if they're advising you to take it NOW to prevent something bad from happening, then take the pill and be very grateful they were concerned enough to prescribe it, rather than waiting for something very bad to happen on the off chance you don't need it =)
Also I did a quick google before posting this and was returned with over 80,000 pages of reasoning and evidence to pour through showing the benefits of using either ACE or ARB Inhibitors to reduce/slow Diabetic related Chronic Kidney Disease - so your statement regarding there being no advice out there regarding the use of these types of medication to treat/slow Kidney Disease due to Diabetes seems a bit premature, there's definitely evidence out there, and it should definitely be followed in my opinion...I *KNOW* for certainty what happens when you don't =/.