Skynet's timeline in
The Terminator movies can be modified. After all,
August 29, 1997 never became the
bad fucking day for everyone not wearing
3 million sunblock that the second movie told us. After all (and I do warn of
spoilers), Arnie tells
John Connor that
Judgement Day cannot be prevented, only delayed.
Perhaps this was done to appease fans of the Dark Horse comics who wondered how John could still be a teenager in the year 2000. But then, Terminator 2 was 1992, eight years after 1984, and yet John was about twelve years old. The plot begins to have more holes than Arnie's leather jacket.
However, Skynet is not wasting resources per se by sending back the first Terminator - as, without the first Terminator, Skynet would never have been built based on the chip design. The first Terminator is, in fact, to ensure Skynet's existence in the future.
The real question arises when you ask whether Skynet ever gets destroyed, eventually.
The Terminator 2029 games by Bethesda Softworks claimed that, once Skynet (which in their view was a space station) blew up, a secondary backup system activated.
Kyle Reese in the first movie claimed that the Resistance had won and Skynet, desperate, sent back the first Terminator.
But Terminator 3: Not Even Fan Service seems to blow these out of the water a bit, unless the one future sequence of John standing atop wreckage with a tattered American flag is meant to indicate they do eventually win.
This in turn raises the question, why does Skynet even bother, if it knows all it will achieve by starting the war is ravaging the planet for three decades?