Gonna answer my own Q (why use mono Topshot). From an article in Salmon & Steelhead Journal, titled Spoon and Spinner Fishing with SuperLines for Salmon and Steelhead, by Bill Herzog:

When you’re attaching lures, avoid the Cardinal sin of tying your lure directly to your mainline. The reasons are many, starting with snagging up.

...when cutting the line is your only option and leaving a section of floating line (that never goes away) ruining a pool for other anglers is the ultimate sin. An 8- to 10-foot section “shock tippet” or “top shot” of mono is the best and only option for river fishing.

...why adding this length of mono is not an option. Hang-ups are simpler to remedy, as the tippet is somewhat lighter than the mainline. What ends up getting lost is the lure, or at worst the tippet and lure. Mono sinks, unlike the floating super line, so revolving lures can be presented at depth faster and at “normal” attitude. It’s called shock tippet for good reason — the stretch of the short length of mono provides a cushion, so to speak, when a hot fish strikes ferociously or makes lightning swift changes of direction without fear of hook(s) ripping free. The rubber band effect of mono shock tippet also prevents a determined head shaker from easily throwing a barbless hook. Lastly, super lines are opaque. While most aggressive salmonids would strike a lure tied directly to a bright yellow line (or an opaque natural toned one), just as many or more turn away at the last second. Numbers of the strikes go way up when a section of camouflaged natural toned translucent mono is tied to the lure. Mono knots don’t slip out and are far easier to tie to the lure.

If I inadvertantly, thru ignorance, violated any copyright laws, or pissed off the publisher, his son or the author -- please delete this post and accept my deepest apologies.

Sincerely,
Roger R