i propose an even MORE rare card
3 
spell
do 99 damage to target player.
it may be OP, but it's SO rare it will become balanced...right?
/sorry for using sarcasm for illustration.
Lol I think I got distracted by a text message and absent-mindfully went to another page instead of pressing post... so I'll rewrite/summarize.
An extreme example isn't really compatible with a marginal one. If I had said the card produces 100 quanta for each element every turn, then your example would be spot on because neither of them are reasonable. The card I "suggested" is a reasonable one because it isn't too overpowered (but that's just opinion). Many other cards have been suggested that were at least as overpowered as this one. If we base it on reasonability alone, the people that suggested those cards would also share my opinion. That doesn't really matter though. The real point is that this card is more reasonable than many other card suggestions. That doesn't really matter either because it doesn't show reasonability on a scale everyone would agree upon. The reasonability of the card is what we should focus on, even if it is just opinions.
In summary, I feel the card is reasonable because it isn't too overpowered, the chances of obtaining one are extremely low, and everyone has an equal opportunity to obtain one (or more). The second part isn't really required in my opinion (universally), but it's a contributing factor towards reasonability in this case. If it were a common card, everyone would just use these instead of regular pillars. Having one of these in your deck shouldn't change your overall deck structure, or you will likely ruin the balance. Sure, if a person happens to draw it during the battle, it will give them an advantage, but that advantage is small and justified by equal opportunity. That's pretty much all opinion though. It's kinda funny that my "summary" is nearly as long as what it is meant to summarize.
Factors that determine the reasonability of a card:
-how powerful it is
-how easy/difficult it is to obtain
-how it is obtainable
If anyone thinks there are other factors, or think anything is wrong with the factors I listed, please say so. I'm looking forward to reading responses about reasonability of cards in general. :-)
Edit: Actually I'm just going to make a topic in general discussion about what factors determine the likelihood of a card being added to the game. So just post there in response the reasonability... which basically means this thread isn't needed. Discussing reasonability is just too off topic for this thread even if it does pertain, in part, to this card.