Game Freak never made an official statement explaining their reasoning, so its impossible to know exactly what they were thinking
(e.g: if Pikachu from set X has a holo bleed, that is worth x much!") Or is it in general a good idea to buy them
Its not an all-star card, but you could potentially pay two mana for a 4/4 flier that cascades into just about anything
This is harder to evaluate ahead of time, but it tends to track with how playable a card is in the format its aimed at
Its worth noting that Ursula, Deceiver of All is also available as a Legendary Rare card, but even in its non-Enchanted form, the average asking price is still in the region of $50
Enthusiasts estimate that the frequency ranges from 1 in 500 to 1 in over 1,000, depending on the set