My vote is yes it is worth the time and effort to install and use the Vault.
Everyone can use common parts.
Content Center is very valuable
Search makes the Vault worth it just by itself.
Backups in a vault have been a life saver for me. I know all your engineers make diligent Backups every night before leaving for the day; ours did not and have lost work.
It is easier to learn the system with three workers, than wait till you have 9 and decide to install and train everyone at the same time.
When we first started SolidWorks back in 1998, using SmarTeam we could only afford 3 seats of SmarTeam. Our Engineers worked on a network drive, without a Vault. The drafters would move their completed projects into the SmarTeam Vault. Then when SolidWorks came with PDM Vault we started using it. What a nightmare getting the entire project list off the network and into the Vault.
If you have the power to get this done great, if not get management to back you with using a Vault.
Standing
AutoDesk Inventor Router 2011
SolidWorks Pro 2009 x64, SP3.0, PDMWorks Workgroup, SolidWorks BOM,
HP xw8600, 64-bit Windows Vista Business, Service Pack 1
Intel Xeon CPU, 3.00 GHz, 16 GB RAM, Virtual memory 166682 MB, nVidia Quadro FX 4600