Not really and instants have so many going on than just counters. Black has a lot of creature removal spells, red a lot of direct damage spells, white has a bunch of removal spells and both white and green have instants to prevent combat damage etc.
Hell, just in Reanimator decks because Entomb (put a creature from your library into your graveyard) is an instant you could cast it at the end of your opponents turn when he's tapped out and follow it up by a Reanimate (put a creature from your graveyard into play, lose life equal to it's mana cost) while you're relatively safe from counters and even if you only have 1 black mana at your disposal.
Then there's abilities of things in play that can be used at any time that can range from letting you look at the top cards of your library and put them back in any order to dealing damage or winning you the game when used in combination with something else (
grindstone +
painter's servant for example) that you may want to wait with to make sure your opponent can't do something to stop it.