If you copy a project, or any part/assembly/drawing from the network, how do you know someone else hasn't done the same thing? Then when you copy it back, who is going to overwrite who's latest revisions? If your group is extremely disciplined, then you can get around these inherent network issues. If your group is spread out and doesn't communicate every day, you will eventually end up having several copies of the same files all over the network and no one knows who has the latest revision.
Just ask yourself one question: How much will it cost if the wrong revision makes it out to the shop floor? $5,000, $10,000, more. These dollar figures are way more than getting a PDM system in to simply manage your SolidWorks data.
These are PDM 101 issues that PDMWorks can address. So if you have the software (included in SolidWorks Office Pro), I would highly, highly recommend you look at using it. I've seen it fix these issues many times with little effort needed from the engineering and especially IT groups.
Steve O