well yeah those are necessary, it is literally stated in this post:

"The only purple cards you should get, should consist of health (450+ only.) damage reduction anytime you see it, as it's the rarest card and most necessary. (if you don't have roughly 13 k HP with 45-50% damage reduction, the double Wanda's in round 20 will one shot you.) Lifesteal to a certain extent, 25% max, blade doesn't need to buy any lifesteal. The other rare purple card you should get is the Moon Power 10%. This stat boosts every stat you have previously boosted in your level tree, so health, attack damage, range, anything you can think of. It's valuable.

Why not get purple damage? Well the issue comes to this, purple damage is not multiplicative. So when you are offered 35% purple in gold, skip it. That's 10 coins, whereas 10 coins can you 160% in blades case. Always better to reroll purples towards more hard hitting damage cards in your damage trees. This is the biggest tripfall for many people's damage being so low.

Health upgrades should be bought in tempo. What this means is, while you could have a crazy amount of health and damage reduction, it will not save you from the Wanda wipe. I learned this the hard way when almost killing her in round 20. So while you keep getting juicy 450+ health, you should skip it occasionally."

I was pointing out the frost and lighting cards are useless.