Being Black and Green, Dredge was in the colors that can most easily play spells or creatures from the graveyard or retrieve them from the graveyard into your hand
[M][M][C][C] Sword Star (VSTAR Power): 310 damage
It's also convenient for providing targets for "Vampire Scourge's" revival effect
Like most Pokmon cards, the value of Porygon depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Paradox Rift)
It is sufficiently over powered that it is one of a few cards I no longer bother to run in my cube
Low curve decks and token based decks are probably the only decks that would want him, as getting 2 permanents to enter the table on your term consistantly isn't free, unless you do bouncy things or generate tokens constantly