Old question, I know, but just wanted to point out that you may need to add quotes around the target workspace path. For example; I tried
C:\Eclipse\eclipse.exe -data E:\Eclipse Projects2 and it would open a blank, default, workspace while doing
C:\Eclipse\eclipse.exe -data "E:\Eclipse Projects2" allowed it to use the existing workspace. I'm guessing this varies based on OS and/or Eclipse version, but I'm not sure exactly what factors into this, so just try both ways until you get one to load the correct/existing workspace.