Come on man...you know what hitlag IS right? Shieldhitlag functions the same as hitlag (even though the values are different). Here's an extremely easy way to see the significance.
Use the timer item and have Ganondorf do a shorthop dair on the slowed opponent (making sure to buffer the dair of course). Now, you should notice that the opponent is still in hitlag while you're not in lag. BINGO, you've just gained a large advantage because of a large hitlag difference
Take that concept and apply it on a smaller scale and you'll see my point.
I was never aware of the differed hit lag values, but I'm still confused about their relevance. The main point of hitlag is to calculate shield advantage (at this time, at least).
So if Marth suffers 6 frames of hitlag, and the opponent suffers 11 frames of shieldstun, the relevance of recording shieldhitlag opponent, when the two run concurrently seems.. redundant.
SHIELDSTUN AND HITLAG DO NOT HAPPEN AT THE SAME TIME! Hitlag (and shieldHITLAG) happen BEFORE shieldstun. They are two seperate events. Ergo, if you have an advantage in hitlag, THAT WILL ADD TO YOUR TOTAL ADVANTAGE. It needs to be added into your calculation.
However, the move having different hit lag values for whether it hits a shield or not is of much higher importance, but I've not noticed such discrepencies doing Marth's frame data as of yet. However, the shield values are in general of much higher importance..
Hence why I say it's of not much importance.
Shieldhitlag and shieldstun are different.
This is how it works.
Marth's dash attack hits shield (not tipped). Marth suffers 6 frames of hitlag while the opponent suffers 8 frames of shieldhitlag, giving you AN ADDITIONAL 2 FRAMES towards your advantage (because Marth is actually out of hitlag 2 frames before the opponent, and they haven't even gone through shieldstun yet). Opponent suffers 11 frames of shieldstun.
Using YOUR method of calculating advantage, you would've incorrectly calculated that Marth's advantage on a shielded nontipped dash attack is -25. However, it is ACTUALLY -23 because Marth has 2 extra frames where his opponent is still in hitlag while he is not (BEFORE shieldstun).
Now, looking at the tipped dash attack you've got
Marth's dash attack hits shield (tipped). Marth suffers 12 frames of hitlag while the opponent only suffers 9, CUTTING INTO YOUR ADVANTAGE. Opponent suffers 14 frames of shieldstun.
You'd have calculated the advantage to be -22, when the advantage is ACTUALLY -25 (you suffer an extra 3 frames of hitlag while your opponent starts cutting into their shieldstun), making it WORSE than an untipped dash attack.
THAT is why you need to take note of the hitlag differences. The extra hitlag by either side affects Marth's advantage when the move is blocked. If Marth suffers less hitlag then the opponent, then he is cutting into his ending lag before the opponent moves to shieldstun and gains advantage. But if the opponent suffers less hitlag than Marth, then they're cutting into their shieldstun before Marth even goes into his ending lag. Every one of your advantage calculations would be off by a couple frames. Though that may not matter with things like dash attack, I'd think it would matter with oh...d-tilt, fair, nair, ect, ect.