VenonUIO
Duke Fishron
You're forgetting that you can have multiple summons very easily by default from potions and a completely free station.
It's a completely terrible omni-class bonus that's almost completely useless in every difficulty above normal once hardmode starts for every class except Melee, but you aren't wrong.
You can have multiple summons, but the summon potions don't last long and are generally only worth crafting if you're devoted to a Summoner playthrough. Even when I did a full playthrough of the Summoner class I only ever felt the need to use them for the daytime Empress fight. The Bewitching Table is more substantial since it lasts slightly longer, can be placed and doesn't need to be replenished, but even that only leaves you with two (or three max) summons, which aren't as powerful either if you aren't sticking to Summoning armour and accessories. Defense isn't a terrible bonus, if you invest nothing in it and don't stack it then you won't notice it, but that's a side-effect of eschewing the mechanic altogether anyway. I never found crit particularly useful to invest in either, considering how random it is compared to straight damage bonuses.
If you got spare accesory slots you can stack putrid scent with those emblems you're using, and unlike the flesh knuckles its acutally a useful accesory on its own. If I already have a mech glove I'd rather stack a putrid scent than a berserk glove. The crit bonus can be used for magic support weapons like the nimbus rod and magnet sphere without having to use a magic main weapon.
No... the less defense you have the less useful extra defense is. Let's say you have 0 defense and get hit by an attack that does 100 damage in master mode. Adding the flesh knuckles would reduced damage taken by 8%.
Now lets say you have 50 defense, you would take 50 damage from that attack, and adding on an extra 8 defense would effectively reduced damage taken by 16%.
The Flesh Knuckles are also a useful accessory on their own, not as useful, but as previously stated they can craft into one of the best melee accessories at the start of Hardmode while carrying on all their objective bonuses to them. I'd prefer the 12% melee speed increase, 8 defense and 10% increased size over 5% crit and 5% damage. If you have a Mechanical or Fire Glove then you're already stacking damage, speed, and size. The Berserker Glove gives you two of those bonuses, rather than just one. The crit bonus can be used for magic support weapons, but I specifically mentioned that the crit bonus isn't universal because it barely applies to Summoner, and switching to another weapon, let alone a different classes weapon mid-fight is a hassle, especially since, one again, if you're not devoted to a Mage build, you won't be doing as much damage.
As for Defense, I know how it functions, but a Summoner would want to have more of it anyway if they were severely lacking, considering how the Putrid Scent doesn't provide them as noticeable a bonus due to being a low percentage based boost for AI that rarely hits the target in the first place. The defense of the Berserker Glove for Summoner is also playing in tandem with the autoswing and melee speed bonus, all of which are more substantial for said class than a 5% damage boost.