Oh, you don't melee as a shadow tank then?
In case you never even played a Consular (or an Inquisitor for that matter), the Shadow AC is a melee-based AC, unlike the Sage AC. Same with the Sith Assassin AC, that's also a melee based AC, while the Sorcerer is ranged.
Strength gives both bonus melee damage and bonus melee crit, just in case it's too hard for you to hover your mouse over the strength attribute on your character window for more than 3 seconds as well.
Willpower gives bonus melee damage, bonus melee crit, bonus force damage and bonus force crit.
Neither directly affects your tanking capability, even less your defense ratings, and both have only a minor effect on your aggro capabilities which are derived from the damage you deal.
Basically, you'll tank way better using a crafted purple strength based shield than you'd do with that blue mission reward you got from Corellia. Even if there was a comparable Willpower based shield, the difference between 43 strength or 43 willpower would be neglectable to a tank. If that's not enough, simply get a critted one, and put willpower in the augment slot.
As a shadow tank, all our threat moves are force based. The only melee move I use is double strike- project, force breach, slow time, telekinetic throw... these are all force based attacks. While double strike is effecient for its damage/force power cost... it has no bonus threat coeffecients and is mainly used to proc particle acceleration... which forces the next project to crit (project in turn is talented for bonus threat). Slow time is -very- force/damage effecient, and generates bonus threat+ is aoe+ adds two debuffs to every target. Breach is AoE and also leaves a debuff (force effecient for damage, no bonus threat however). ST/project both stack harnessed shadows... which increase TK throw damage. at 3 stacks also causes TK throw to do self healing...
As far as "directly affects tanking ability" Doesn't matter what my defensive stats are, if I don't have threat I'm not tanking. As a new shadow tank (what you are talking about) threat can most definately be an issue- especially when one is getting a feel for best threat rotation. Even guarding a geared dps, the threat difference can be too much (like when I started doing FP's as a fresh 50 with 2 geared smugglers.... I learned threat quickly O.o)
TL;DR Melee crit is a -very- insignificant part of a shadow tank's threat output. Our only melee skill (other than lol 0 force attack for filler) is used ONLY to proc a more powerful FORCE attack. Everything else (force attacks) are our real bread/butter.
So no.. in fact you DON'T need melee as a shadow tank. In fact I can/have successfully tanked using only my force abilities at 10m range when the situation demands. yes I do suffer a threat loss (small) but only because I can't use my 1 melee ability to proc tasty project crits.
The wise speak only of what they know: please learn a class before talking down to someone about it.
That being str WILL improve threat for a shadow/sin tank, but very marginally compared to WP. The oversight of missing a craftable offhand/matrix cube for us... while annoying at best, is not fair.
Shadow tanks should not be forced to take sub-par gear due to a 2 month (since live) developer oversight. Combined with the fact that there is no willpower/defense based matrix cube... and its a rather harsh slap in the face for Shadow/Sin tanks.