Every character has access to Guarded Position, so if someone is in bad shape, they or someone else in the same engagement can spend their action to add extra difficulty dice to attacks. Two extra black dice doesn't reduce the "to hit" chance by a heck of a lot, but it does greatly degrade the overall quality (mostly damage) of the average hit. They could also use any remaining Fortune Points to pull recharge off their Active Defenses. Imperiled characters have a lot of defensive options, and piling on the black dice works pretty well in my experience. The system works pretty well as-is once the players realize all their options.
For example, unless they've got really low Toughness ratings, or taken a lot of fatigue already, most PCs can just run the heck away. A PC that takes 4 fatigue to disengage and run to Long range (and then does Guarded Position or Assess The Situation there) is suddenly a much less interesting target, as most NPCs have to pay 4 wounds just to reach them. It's far from guaranteed (a Nemesis or a high Wound Threshold enemy might pursue).
(Also, you might encourage them to buy the bonus defensive reactions. Armour Expertise, Bodyguard, Celerity, Dirty Tricks, and Ward can all penalize incoming attacks without costing you your own action for the turn. The same is true about Bloodsplatter if you're in Conservative Stance and good at killing things.)
The existence of Guarded Position (and all the various reactions mentioned above) makes it a little dangerous to add house-rules allowing players to somehow restrict targeting. If one PC can hide behind another, and also use Guarded Position (or any of the other Actions that let you boost the defense of a nearby ally) on their whole engagement, you can quickly end up in a situation where the only valid target has a ridiculously inflated Defense rating.
I'm not saying "don't do it" necessarily, just that if you are going to include such a house rule, you should probably get rid of Guarded Position (and Improved Guarded Position, and then take a good look at Bodyguard, Assess the Situation, etc) so that you're not creating an easily exploited "turtling" loophole. What you _don't_ want is multiple characters in every engagement holding back and using guarded position to make the group invulnerable, and manoeuvring to give an assist to the only character that's actually attacking. That's a bordering-on-viable tactic with the rules as written, and would definitely be game-breakingly good if the PCs could (on top of all that) choose to make their highest Defense & Soak character be the only legal target. Proceed with caution, and don't just blindly trust that your players won't abuse such a loophole. If they're so concerned about a perceived loophole that you've already told them you won't abuse, it kinda implies they'd be strongly tempted to abuse similar loopholes if the situation were reversed.