Sure. The safety slide is designed to keep the line in front of the bow so that it never gets near the bowstring where they can become tangled. This is when there is a danger of "snap-back".
With a spincast, one of the main problems is when the shooter forgets to push the button to release the pick-up pins in the spinner head. This can and often does break the line, losing the arrow and/or the pick-up pin is sheared off, making the reel useless.
To help prevent this, many spincast users tie to the back of the arrow and take all the slack out of the line when they nock the arrow on the string. This way if they have forgotten to push that release, the nock pulls off the sting as they draw the bow.
When you use a safety slide with this set up, and youreel all the extra line up, the slide is only about the middle of the arrow. You can still draw the bow w/o knowing if the button/release has been pushed. Been there, done that. (see above and add miss the fish to that list)
What the rod extension does is put the slide out to the tip of the arrow before you draw. Again this will pull the nock off the bowstring if the release is forgotten.

Hope that helps.