The third hit(down) still works as a pseudo spike, if I'm not mistaken...
One of my favorites to do is forward+b(initiate), forward, down, then end it from there:
-If the opponent's pretty far, end it with down
---To add to that, if the opponent's near/off the ledge, end it with forward to knock them off and start edgeguarding
-End it with forward for decent knockback to respace, usually reliable enough
-If the opponent's really close, end it with up to pop em up for potential juggles. Especially useful on Snake imo
Also, about that third hit, according to Emblem Lord, if you end the combo from there, Marth will return to about a half step from his starting position. Helpful if you've been hitting someone's shield so you could be safer from retaliation.
If I'm just feeling lazy, I could just do forward, forward, forward, forward.
You're right in seeing that it's situational. The forward x4 one is most reliable if you're lazy or don't want to move that fast, but the extra options do help. Just remember to watch their DI as they try to escape from it - if you notice a pattern in their DI, change it up to see if they fall into it.
Remember that the last hit of the up combo does kill(~170-180% for most characters I think).
Against some opponents, you can try to trick them by suddenly ending your dancing blade early, though I wouldn't recommend it too much.