Dair is neccessary. You can break off into many combos using dair.
For example, if you hit them with dair at a % where they lift off the ground, you can put them in unavoidable combos such as dair + uair + zair, dair + usmash or dair + neutral B, which often kills them pretty much. Against smaller characters, not only is it needed, but it is pretty much the only way to combo.
Which brings me to Usmash. It is, in my opinion, Samus's best move after zair. Not only is it one of the most powerful, but it knocks them slightly into the air, meaning its a great combo starter. Utilt is garbage btw, don't use it, ever. Dtilt is where its at, it has better range than Utilt and it is a good finisher. Dthrow also can lead to better combos than Fthrow. Dthrow--->Uair is unavoidable. The only point of the match where you should Fthrow is when the opponent is near the edge, which puts them in a good position for a spike.
I dunno if this stuff was already mentioned, I just don't feel like reading 4 pages -.-