There is the 80/20 rule and the 20/80 rule. Basically, back in the day, the thinking was 80% of the traffic should be local and 20% of the traffic should go remote. So for example, most of the traffic would be between computers on the same vlan/subnet, and only 20% of the traffic would go to other subnets, like the server farm or internet.
These days the rule is the 20/80 rule, which is basically the opposite. Now only 20% of the traffic stays local to the subnet your host is on, and 80% of the traffic is destined to other networks. So for example, with cloud computing and so forth, most likely your computer does not do much talking to other workstations on the same subnet. Most likely it is connecting to other locations, like websites on the internet, cloud hosted applications and so forth.
Ultimately a vlan is only locally significant (to a switch) but you can trunk it to other switches. The way the networking world is going, basically you do not want to trunk vlans unless it is really necessary, especially between the distribution and core layers. The reason being, if there is an attack or virus outbreak on an access vlan, you do NOT want that issue to spread to the rest of the enterprise through your core and distribution networks.
By isolating the access vlans locally, you comparmentalize any network problems so it doesn't spread.
Hope this helps!
Andy
