No I understand. Thats actually why i asked about the spike, becuase I didn't know how powerful it was. If it was this crazy 'dead at 0%' spike that was just notoriously hard to land, then I would insist it's worth learning to use, but if it's really not even a strong spike to begin with, then I understand.
The reason I would prefer the DB spike over dair is that a dair off the stage (with significant distance off stage) would make it impossible to recover. The point would be to stall your descent while at the same time spiking the opponent, then turning around and recovering. Furthermore, a bair, nair, or fair, if diminished, would just knock them further up in the air a lot of the times, and as a result they would just be attempting to recover again. Although I know marth is a fantastic ledge guard, I find it quite worth it to try and find a way to prevent them recovering at all.
I understand what you're saying, and I trust what you're saying, but for the sake of discovery, I'm going to go ahead and attempt some new stuff, mostly cause I don't have enough experimenting going on to begin with.