My question is why dial in strings even exist? If they don't have a way to cancel between hits for a mix up or plus frames for more pressure there's no point.
I'm not a FG dev so I can only guess but... my guess is that it's just one more tool they use to try to enforce more balance. As in either (a) the string itself would be OP if you could stagger/cancel between every hit; or (b) they just need a way to "ding" the character a bit to balance out other strengths.
Like with Kitana, her b2 alone is very good: long range mid, pretty quick startup for that much range, pretty quick recovery, and 0 minus frames on block. It's a great neutral check and stagger on its own. My guess is that the devs said, ok, we're going to balance how good b2 is by making the b231 a dial-in string, so it's a little easier to defend against. If she could stagger each hit, the pressure she gets off that string would probably be broken.
Who knows if the devs knew going in that b231 would be punishable on hit by jumping--but they certainly know it now and they haven't changed the string (and Kitana has gotten a lot of buffs). So I tend to think they
want her to take some risk if she throws out the whole string in the neutral.
Spawn's 24u32 string is kind of similar. You can stagger 24 all day and get a great mind game out of it (is he going to finish the string or not?), but if you do finish it to fish for the KB and get the plus frames, you also have to commit to the risk of being flawless blocked. The dial-in part makes the string more balanced.