No, Open Source does -not- mean more exploits. Typically in the FOSS world, an exploit is usually fixed as quickly as it is found and fixes are usually available before any "malpractice" is carried out.
The benefit of having lots of developers available. By contrast, fixes come from Microsoft more slowly, although things are much better in that respect than they used to be.
If anything, security through obscurity is practiced in the notion that closed source = more secure due to not being able to see the source.