I am comparing "PC with Barbs" to "PC without Barbs", not "Use Barbs" to "Not use Barbs".
A PC with barbs lands spells 75% of the time that a PC without landed 50% of the time at a cost of 0.5 level 1 slots and reactions/cast.
That presumes such a PC will always use barbs. If you make the decision to use it before the die is cast this is true, if not, then it is not.
This is far, far weaker and far less likely than stopping a critical hit IMO.Suppose you find a weak save and a spell the monster won't want to land. As an example, synaptic static and an int save.
The monster has a 2/3 chance to save. So you throw out static; 1/3 of the time a LR is used, and you are done. 2/3 of the time, you barb and force a reroll; again, 1/3 of that leads to a LR use.
Except LR is measured in integers and 1.67X1/3 is still less than 1. Moreover you will need to use multipleYou go from 1 action 1 5th slot for 1/3 LR removed, to 1 action 1 5th 1/3 reaction 1/3 1st for 5/9 LR removed. That is 1.67x as many LR removed.
Or if it was "needed" now you can't cast absorb elements when the legendary monster breathes on you or shield (or barbs) when he bites you and you die from it.What more, if there is more than one spellcaster (or non spellcaster save inducer), and your barb wasn't needed, you can use it in their spells/effects.
You are burning your reaction and likely doing it to no effect.
I is per round, not per turn because you only get 1 reaction per round.While 5/9 LR per turn isn't a lot, 2 spellcasters are now stripping 10/9 per round instead of 2/3, and using 1st level slots instead of high level ones (how many 1st level slots produce effects a mosnter eould burn a LR on?). So around turn 3, at an average of 30/9 LR burnt, the monster starts risking using up their last LR on each effect. So you can start landing debuffs.
The last time as a player I fought enemies with LR (fighting 3 dragons from a cloud giants flying castle) they used LR against ensnaring strike to keep from plummeting to their death, and while that was a corner case, things like charm person, ensnaring strike and cause fear can commonly be very debilitating against a single enemy. Fighting a Vampire for example, synaptic static will cause damage that will be mostly healed on his next turn and result in a -4 or so on attacks until he saves. Why would I ever use an LR on that? Cause fear on the other hand will stop him from attacking completely until he saves (unless the party is stupid enough to walk up to him). Similarly, ensnaring strike will stop him from attacking and make him use an action to escape.