The reason a RSV day credits 4:12 is because 75 hours divided by 18 work days is 4.16, which rounds to 4.2 which is 4:12. Yeah, if we could negotiate 15 days off for reserves, that'd be sweet, and the math would work out to 5 hours a day for reserve. Negotiating a 5 hour pay for reserve and getting 15 days off as a result is doing it backwards. You go for the days off, and a reserve day "credits" whatever the min guarantee divided by the days worked is. The 4:12 "credit" normally only comes into play on hybrid lines or PTO situations. If I actually fly on a normal reserve line, I don't get paid OVER 75 hours because I flew more than 4:12 every day they used me. That DOES happen on a hybird, though, because the 4:12 credit is used to build the total credit amount of the line. Let's focus less on how much a reserve day "credits" outside of a hybrid line and more on just getting more base days off. I think every one of us would give up PRL days for a hard 15 days off a month. I know I would.
ASB, as has been said, is a slippery slope that has the potential for serious abuse without the proper contract language to go with it. Just ask the FAs. Personally, I'd rather not see it AT ALL. If they company wants to negotiate for it, we need to make it a VERY, VERY expensive negotiating item for them. Like 8-10 hours minimum above guarantee + per diem for each ASB assigned expensive. Imagine the abuse ASBs will get during IROPs as they try to recover. I could see them taking every SCR guy on the grid and making them ASB if they could. HCR, for me, is good enough. The provisions in the FSM make it fair, and I'd much rather be sitting in a hotel room they pay for rather than sitting at the airport, which can be even more fatiguing than anything. Nothing like getting that call RIGHT before your ASB shift is up to go do 6 hours of flying.
I don't THINK they use HCR much as it is because it has to be assigned ahead of time. The only time I've ever seen it was during a snow storm my first year in BOS. Give them ASB and the ability to escalate SCRs like they can the FAs, and we're setting ourselves up for abuse.