I read a couple of Q&A's here on vlans, but mostly they seem to assume having VLANs as a given.
My situation is that I have inherited a network of about 6 different subnets [some for different companies], and all the end devices are simply plugged into a total of around 20 HP Procurve switches and they all come together into a (Routerboard) router.
At the moment switches are used as dumb switches -- no VLANs configured, no nothing, and all the net division is regulated by router firewall/nat rules.
On one hand this is very easy to administer -- I don't have to do anything with switches and can make all the changes in the router.
However, having everybody assume VLANs as a given, what would be the disadvantages of the current setup and should I consider enabling VLANs in switches? Why?