But you have to keep in mind that a Barb is going to pass it more often then not, and even more often as he levels up.
At 1st level assuming a Con of 14 (it should range between 12-16) its a +4 save so 11+ needed. 50/50 chance.
At 10th level assuming Con 16 its +7 (meaning an 8+ needed). 65 percent.
At 17th level with Con 20 its a 4+ or better or 85 percent.
Also you'll start to see things like getting Bardic Inspiration and Bless right before the fight ends so he can use them on his Frenzy save. If your ok with that then its no big deal.
Both resources that could be used elsewhere. Im OK with this.
A player in my game is constantly trying to use
guidance whenever the DM calls for a skill check (even on perception checks to notice things as we walk along). As a fellow player, I had to tell him to stop. Unless its obvious in game that the PC needed guidance, then its immersion wrecking gamism to even suggest it.
But the only thing that I don't like about it will be the stuff that happens outside of combat. things like Barb needs to make a check like insight/survival/persuasion or whatever, and they decide to go into a rage before they do it, which would be RAW.
This gets me back to the guidance thing above. Entering rage, to get a bonus to perception assumes your character knows you're making a perception check at that moment. Its metagaming of the highest degree. I mean you're walking down a road, and then you burst into rage for no other reason than the DM calls for a perception check, or a survival check.
You make a survival check to represent hours worth of navigation or foraging. The perception check is made to notice something that you dont know is there, so you cant rage in response to it to improve your chances of noticing it. Most skill checks are the same. Int checks to know something, Insight and perception checks etc.
Putting that aside, I have no problem with a barbarian raging to improve his chances to smash down a door (no penalty to Str checks), Intimidate someone (no penalty to Cha checks) tumble (Dex checks) etc. Most active skills its entirely appropriate.
Plus if you're desperate it means your barbarian isnt shut out of everything barring combat (social and exploration pillars) on account of frenzy.
And what barbarian in their right mind is going to do this anyway? Raging to pass a skill check? I mean there are rogues for those things.
It's true that it burns a resource and it creates hilarious things like 'rage investigations' or 'rage Medicine' checks, and even kinda appropriate 'rage intimidation' or 'rage performance' checks. But it feel a little TOO loophole-y, and to write in a patch starts to make it too complicated for my taste.
There's no loophole. No DM would allow it.