A numerical rating for cards wouldn't make sense since you have to factor in multiple factors such as which planeswalker you're playing it in, what the current mana gains of the planeswalker is, what complementary cards to this card are also in the deck, what contrary cards are in the deck, what the meta is like, whether you're going for a control, aggro, creatureless, anti-creature or whatever build.A simple number cannot adequately compare all these factors. A linear card ranking list would have the same issues.
It's not too hard, with a spreadsheet that can average everyone's rankings I feel that a 1-10 ranking can take into account all of these factors. In some cases, a ledger would be handy to notate why this ranking was given (ex. for ToA black node. seriously, I have a whole set of cards that is only for that). Very few cards will be considered a 10. Only the absolute best, most indispensable cards. From there, a 9 will be a card that is super versatile and synergetic in many circumstances, and a card that you often want to include in most of your decks. And the list goes down from there. For people that don't have all of the (dual-colored) Planeswalkers, their experience may vary with some cards, but the point is for it to average out with many people's input. I worked in Marketing Research, All feelings and opinions can be expressed on a 1-10 scale hahaha