Proving this would be difficult, especially in pvp, and without a specific gear level target, but what would you compare it to?
The increase going from 50% to 75% offhand would be worth about 15% of white damage (1/7). Assuming white damage is half your damage, that means it is worth about 7% total damage, or about a percent and a half of total damage per point.
Aside from that, it scales well with Precision, Serrated Blades, Blade Flurry, Deadliness, and gear. If not gouging, snd is almost always my first cp, even on a player.
You could indeed macro a weapon swap on your Ambush, such that you Ambush and immediately switch weapons without an additional cooldown. I had it both ways.
I might want to double ambush, or think I might otherwise vanish soon enough that I should not switch.
Swapping also normally resets your swing timer, so sometimes you may not want to do while actively wailing on someone (many people fell to ambush plus one hit from my giant offhand sword, I usually saved the switch for my first gouge or loss of control such that it was free)… or did it immediately after that giant offhand sword connected. You have a moment as you just need it before hemo, and you just spent all your energy on ambush and snd if you are staying in.
Keep in mind that players were not made “elites” until BC. A crit ambush is reliably half a casters health. I had a rogue and mage at 60, and my rogue could very much one shot my mage with a good Ambush (I was mostly in “of frozen wrath” gear, not hp gear). This is a best case ambush build, so a squishy target means you can be back in stealth in close to the stealth cooldown. If your ambush does not crit, you can gouge and attempt restealth.
You can also snd + bf, and try to power on the damage, with hemo helping you build enough cp to make finishers manageable, or lay an expose armor on a 10k health sl lock and chew them up faster than they thought possible as you have decent sustained damage.
Everyone has the “best” pvp build though, as they are highly linked to play style.
I like speed and options, so I like sub / combat. I also liked ganking at LHC and uneven fights, and in those cases you want to be a razor. If you want to take that guy who just barely wandered out of range of the group playing find the rogue, you had better do it fast.
PvP was much faster and more brutal than it is in retail, there was a lot of two shotting going on by a lot of classes (hunters, warriors, and mages being the other notorious ones), and this made stealth a lot more valuable.
You do not want to be caught half way through working some complicated ritual dance on some guy for like half a minute when his mage or hunter friend shows up. You want to be in stealth, so you can power into that mage (if it takes more than a few seconds you are not going to win that fight regardless of spec), or avoid the hunter.