I totally agree with you that the factory installed heat shrink on a connector is basically fool proof. However, I have found the factory installed shrink is usually only readily available on certain size loop connectors, not all sizes. I have found it difficult to locate it for spade connectors (does it exist?) and in the smaller sizes. Therefore, the alternative method has been a very good option for me. I find the efficiency of time in buying a stock of various size connectors and appropriate heat shrink at your local marine store and using the T-9 (or alternative) fills the bill.
I originally used all loop connectors (you know, the mantra, they will never "fall off"), but the extreme gymnastics of big hands, limited space, small screws/connectors with the accompanying frustration and time to completely remove a fastener on a hard-to-reach terminal block, putting the screw through the loop and getting that tiny screw to take is just too much (I'd probably still be in the boat yard). Unless you have noticeable or extreme vibration, which is not common on sail boats, or on large wire connections that use lugs, the spade is the way to go - loosen the screw, slip on the spade and tighten. One thing I didn't previously mention, once the completed connection screw is tightened, I again spray the entire assembly with T-9. Done. I look at some of the work I did years ago and it looks like I did it last week. Your choice.