The strong hit of Dolphin Slash comes out before you can reverse it, so reversing doesn't make it any stronger; you don't have to do it to kill. You will need to reverse if you're going to fall offstage and kill yourself in FallSpecial though, or to position yourself for some other reason (to edge cancel, for example).
It can be used as a replacement for dair in most of the same Ken Combo setups, though the timing of your jump and your spacing often needs to be slightly different. In some cases dair can connect and Dolphin Slash can't (when the opponent ends up near-directly below you or low down and far away from you where only dair will reach them, which is pretty common), and there could be some cases were Dolphin Slash can connect and dair can't (though I'm not sure what they are, if any exist).
In NTSC, a Ken Combo is almost always better, since it usually kills earlier, you can recover from it faster/from further offstage, and if the dair doesn't kill it puts the opponent in a worse position than a Dolphin Slash (far below the stage as opposed to near the top corner, which is especially important at low damages or against floatier characters). Situations that come to mind where Dolphin Slash is better are: when the opponent can land onstage from a dair (possibly dependent on DI) and survive it but a Dolphin Slash will hit them offstage; and when you're offstage and high up near the side blastline, where a Dolphin Slash will kill but a dair will allow the opponent to recover.
In PAL you need to fair->Dolphin Slash instead of Ken Comboing most of the time (or settle for double fair, or don't go for the setup at all). You can Ken Combo, but if it's not right next to the ledge (both vertically and horizontally), then it's going to be meteor cancelled out of and recovered from before you can do anything about it (the opponent can meteor cancel well before the dair endlag finishes); if you're close to the ledge then you can land and continue the edgeguard, and the opponent has to recover from below the stage. In these situations you'll usually still want to Dolphin Slash if you have the choice, but you can work with the dair, and sometimes dair is still your best option after a fair (usually because it's your only direct followup), so you have to work with it.
Besides that, it can be used:
• In tech chases, since it can be done out of jumpsquat (and therefore out of dash). Because of how long and fast Marth's dash is, you can cover a lot of options on reaction with this. DruggedFox uses it quite a lot.
• As a replacement for sourspot forward smash in various situations (punishing up-Bs onstage from Sheik, for example), since it sends at the same trajectory but is stronger.
• Out of back throw on fastfallers if the opponent DIs inwards at high damages. This sets up DI traps, such as with forward throw near the ledge facing offstage: in this case, DIing inwards will avoid the forward throw followup but not the back throw followup, and vice-versa for DI away.
• Out of shield. Because of the huge endlag and the fact that it consumes your double jump, it's mainly useful as a confirmed punish on very laggy moves (such as smash attacks) when the opponent is at a high damage (and therefore can't punish the Dolphin Slash on hit). It's somewhat similar in application to Fox's up smash out of shield.
• When the opponent is high up and nothing else will reach, using the late hit at the tip of the sword (at high damages where you're not at disadvantage on hit). Mew2King has been using it recently.
