Isn't that 17.5 hp added to every member of the party though?
Yes? I didn't mean to imply otherwise. I was considering the impact on any given character.
At 2nd level, Song of Rest* + Healer + Cook's Utensils is a full-heal-up for fragile characters (like a Con 10 Wizard) and nearly full for all but the beefiest characters (e.g. a Con 16 Variant Human Barbarian with Tough has 29 HP, but that is hyperfocusing on high HP.) Of course, taking a specific class, race, and feat
solely for this purpose, I should bloody well hope it can keep the team alive. I even said that was pretty good.
But even by 5th level, you're looking at doing less than half the Fighter's HP. That's a precipitous fall-off. By level 9, even the Wizard cannot rely on this combo to keep going, contra the poster's claims that there is no need to have healing spells because these things make such an enormous difference. Getting less than a quarter of your HP if you're beefy, and less than half of your HP if you're squishy, is woefully inadequate healing. You can, of course, also spend more HD, but that decreases the efficiency of this method, lowering its impact. Now couple that with the fact that most groups don't take more than two short rests a day, but regularly are expected to endure "deadly" combats that should peel off large chunks of HP (especially from the beefy characters.)
Also, I think I miscalculated the HP gained slightly, so I'm going to re-do my calculations. Somehow I either thought Cook's Utensils added a d6, or I was counting all hits of Song of Rest but only one hit of Healer or...something. Either way, there's some excess in there I need to correct for. With two short rests plus having completed a long rest, you can use Healer three times per party member, while CU and SoR can trigger (at most) twice because you only have 2 HD, from third level on you can get them three times (but only if you're fully rested, otherwise it's back to only 2 times as you get half your HD rounded up when you long rest.)
So Song of Rest, if used at maximum efficiency, gives +2d6 total on the first day of adventure, and +1d6 every day thereafter (because you only regain half your HD while resting and you must spend HD to benefit from SoR.) Likewise, CU requires you to spend HD, but it applies to each one spent, unlike SoR which only adds one die per rest no matter how many HD you spend. So that gives +2d6+2 for the whole (first) day and +1d6+1 for every subsequent day until you spend a whole day not using any HD.
Healer always works because it's independent of HD, and gives a total of 1d6+4+level (of the recipient.) So, at 2nd level, that's 1d6+6 per rest, for a maximum of 3d6+18 per day.
Collectively, that's 5d6+20 or about 37.5 HP added for an entire day. Averaged over the three rest periods (one LR and two SR), we get about 12.5 HP gained per rest, assuming all HD are available to spend. If we bump up to level 3, this adds another 1d6+1 (SoR+CU) and +3 (three hits of Healer) for a total of 6d6+24, average 45 HP added for the day, or 15 per short rest.
As noted, at these low levels, this is pretty decent. A level 3 Con 10 Wizard has 14 HP, while a level 3 Con 14 Fighter has 28 HP. The former is getting on average a full heal each rest, while the latter gets about half their HP. Decent, assuming you can afford the ~5 gp per rest cost of those healer's kits. Much less expensive than potions, to be sure, but spending ~15 gp every single adventuring day still adds up pretty fast. For a level 3 adventurer AIUI, 105 gp is no small expense, and that's what you need for a week's worth of uses if you have a five person party.
By level 9, the total bonus healing only rises to 3d8 (SoR) + 9 (CU, if all HD are available to spend) + 3d6+39 (3× Healer.) That's a total of 13.5+9+10.5+39 = 24+48 = 72 HP at absolute maximum, or 24 average per rest. By comparison, character HP should approximately triple from level 3 to level 9; this method now restores about 1/3 of the Fighter's total HP for the day with each rest.
Now, I admit, I am neglecting the actual HD here, this is only the bonus HP, not the full daily healing, but those HD are present regardless; it feels like more of an apples to apples comparison to consider only the contributions from the proposed things. So, by sinking your race choice, a feat, and your class choice all into stuff that enhances rest-based healing, you can give the whole party (pretty much) full HP after every short rest at level 2, and restore between one third and two thirds of their HP at middle levels. At the point where most campaigns apparently wrap (roughly 11th level), you're giving them at best half their HP, and that only to really squishy characters like Con 10 Wizards.
So yes, I stand by what I said. This is only powerful (and only functional as a replacement for healing spells) if you know the campaign won't run very long, at most level 5-6. If you expect it to reach the teens, or you start out at a higher level than 1st or 2nd, I would not recommend it, and most guides won't either, because they look at a longer view (going to the low double digits and possibly higher.)
*My phone initially autocorrected this to Song of Rear, which I find hilarious.