It may be less complex to have your network as one big flat VLAN when there is very little traffic and no problems...... but think of what will happen if you have one of your 1500 users with a problematic PC or software generating lots of broadcasts. How would you access your management when your one and only VLAN is blocked and your switches are unreachable.
By having a separate VLAN for management you have a much better chance of being able to access your switches during a broadcast storm or other problem.
Even if all your users are equipped with the latest and fastest workstations there is significant reduction in workstation performance if they are connected to a large flat network where they are processing every broadcast seen by the entire company. Cisco recommend no more than; 500 IP , 300 IPX, 200 Appletalk, 200 NetBIOS or 200 multi-protocol, devices on a flat network due to workstation cpu utilization during broadcast handling.
Routing updates should be tiny compared to your background network traffic generated by 1500 workstations.
Take a look at the CCDA/CCDP Cisco Press books and details on the Ciso web site under the headings of Internetworking Design Guide within technical documentation.
Best of luck !
