They were my ONLY Pokemon in my entire deck. The original strategy was to use EXP share as energy acceleration so when one is KO-ed, you can move fighting energy over to another Terrakion. But once "Tool Scrapper" was printed, you can easily discard opponents tool cards to their discard pile. So the deck was no longer viable.
Shaymin EX is probably the only grass Pokemon currently which is used frequently in tournaments.
In recent past formats Shaymin Lv. X, Leafeon Lv. X, Jumpluff (from HGSS), and Vileplume (from HS Undaunted) were viable (though Vileplume was only useful as a support card, not as an attacker), but those four cards are now rotated out for official play. You most certainly can use them for casual play with friends, however.
Also there have been many formats that Fire decks have been really popular such as Reshiram/Emboar and Reshiram/Typhloison, so running grass decks is pretty much a no-go due to weakness. Instead people would run some water variant to counter them !
Fair do. I myself like the Grass type above all else so I will use them as much as I can. I use other types in the deck to help back them up. The system works well and the Water allies are a real pain for my fire opponent.