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The question of using only part of a cheap ticket instead of all of an expensive one has been discussed at length here. Bottom line is that, as soon as you miss a leg, the rest of the ticket will be cancelled. Some people have reported getting exceptions in unusual circumstances with advance explanation and permission, but this is rare. Many others have reported not being able to.

So, if a round trip ticket is less than half the cost of a one-way, you can use the first part and toss the second - if you don't do it too often with the same airline. There are reports of airlines coming after people who do this a lot, or after travel agents who sell a lot of tickets that end up being used this way, to collect the fare difference they thought they had saved.

The logic, or illogic, of this airline policy has also been debated here ad infinitum.