Turtling in my opinion is very effective and not just simply in RTS's. However it is even effective if not more effective in more fast paced games.
To clarify turtling is a way to built a near impenetrable defense and watch your opponent dedicate 60-90% of their efforts, resources, and commitment to one all out to attack to hopefully wipe the turtle. However when they have exhausted all of their abilities is where the counter-attack takes place. Now someone above said something along the lines of "Where's the I win part?" But that was answered as well.
Turtling slows down the tempo o the game, and that is when Turtling is at most effective. People who play RTS's casually normally don't enjoy spending a lot of time against one opponent, in fact they would much rather obliterate their opponents in the most speediest, aggressive, and easiest way. So when a turtle comes along and puts them in their tracks its becomes a scenario where the 'rush tactic' fails, then they must built up their forces and try again, then build up their forces and try again until they can find a weak spot.
Someone who is an expert on turtling should be able to nullify any and all different attacks from multiple different angles and even cover their defenses. This then leads the opposing player into something I like to call, "The Impatient Zone." Happens in Poker too, and any game where someone can survive by dedicating the least amount of effort, or resources against an opposing player. This is the turtler's goal to lead their opponent into this zone so they launch an all-out-attack in an attempt to break the turtler's defenses.
Once the turtler defeats this offensive rush, is when the turtler has no exhausted his/her opponent's resources enough for a decent enough counter-attack, and win. Simply because in a battle of attrition, the one who needs less water and food will win.
(This does not apply to Starcraft/Age of Empires)