Was Weapon Rack’s design based on Serrated Arrows? It’s a cool mirror to that card.

Not directly, but indirectly.

By your definition of parasitic, Evermind was the only "truly" parasitic Splice card. You can cast Harnessed Lightning or Glacial Ray as a mediocre removal spell, but there's not much point in doing so without other energy affects. (There's nothing wrong with Energy as a mechanic; I'm just nitpicking.)

There’s functional parasitism and optimization parasitism. The former keeps the card from fully functioning without the use of another card from its subset (unique to the set). A card that produces energy but can’t spend it has a part of the card nonfunctional without another energy card (one that can spend it).Optimization parasitism involves cards that can function completely by themselves without the need of any other cards, but because of synergy between cards can often be better optimized playing other cards with the mechanic.For example, a card that produces and uses energy (assuming the use is equal to or less than the production) is a lot like a Serrated Arrows type card which comes with a limited number of uses. Energy is mostly optimization parasitism. The designs allow you to drop just one energy card in a deck or a packet of cards rather than always having to play a whole deck of energy cards.

I was re-reading your column archives and stumbled upon this, from the article "Here Comes the Sunburst": "During Mirrodin design we came up with various Serrated Arrows tweaks. The card even led to a major mechanic that got cut during design (although one I expect to see the light of day eventually – it's good to be lead designer)." Has the major mechanic seen the light of day yet?

Not yet. I do like it though so I believe we’ll see it one day probably tied to a set with a strong artifact component.