Marth combos might be circumstantial, but they've been *figured out*.
Just watch m2k vs spacies for your bnb combos, he does it right.
Falco combos haven't even been figured out yet. Just because you *can* get away with dairing repeteadly does NOT mean that is anywhere near to ideal. It's way easier to get close to ideal with marth because arbitrarily up tilting and upairing is actually somewhat similar to the ideal combo.
Be realistic guys, just because falco can do better by facerolling doesn't mean the optimal combos are faceroll. Marth is harder to do well by facerolling with, but his optimal combos are pretty close to his faceroll ones... you just have to make slight adjustments.
Sheik? Come on now; I'm pretty sure KK even made a thread that was like "Here's how you combo marth, guys." There's also a pretty similar outline for comboing other floaties, and fastfallers is just generic tech chase ending in dash attacks to fair.
Edit: What winston said at the end. Exactly. Falco combos are more about decision making; falco's decision making tree isn't optimized yet. Marth and sheik's? It's a lot closer to being optimized, and you can usually make an objective assessment on what decision would have been better. Look at chainthrowing on FD, or tech chasing. That **** has been perfectly outlined for *years*. Falco comboing fox on FD? Not outlined past 40%. Is it better to try to land a dair then tech chase, or go for a fair/nair -->dsmash/fsmash, or dair immediate uptilt, or go for a waveshine, or upair, or jfsoaijfioasjfiojaofa. There's a huge difference, don't be biased guys.
I'll reiterate; just because sheik/marth combos could be optimized a bit better doesn't mean facerolling isn't closer to the optimal. You're fooling yourself if you think falco's optimal combos are dair-->dair, but it wouldn't be too far off from the truth to say marth's combos on spacies are uptilt+upair.
Second Edit: Yeah np foxlisk... I figured you knew what I meant -.-