History, rev tracking, searchability are all great reasons to have PDMWorks. For the price, you can't beat it for this functionality. Bradley is definetely correct - managers always want to know the cost vs. benefits. Most effective argument is to figure out time lost in:
-recreating lost documents
-retracing your steps in the design process
-real cost of incorrect drawings being sent to the shop floor
-time lost looking for documents
Estimate how much of this would be gone if you have PDMWorks, apply a conservative hourly rate and compare that to the cost of PDMWorks. Voila, you have your ROI.
Also - if you have multiple engineers working on some common assemblies or parts, it's easy to justify PDMWorks for collaboration. With a network, if you want to make a change to a part you have to open up the entire assembly to save it, making the entire assembly inaccesible to anyone else. PDMWorks let's you truly collaborate on this work.