Drawbacks:
There is a big list, I will give you a few of the main concerns.
1 Broadcast Domain size. The quantity of broadcast traffic generated by 600 nodes in a single broadcast domain would be enough to impact the performance or every device on the network. you would have a good chance of seeing packet loss on any 10 meg 1/2 duplex switch ports due to buffering problems. The issue with the broadcast traffic is not bandwidth utilization. The problem is that all interfaces on the network must process a broadcast high enough up the IP stack to ensure that the packet is not intended for that node. This packet evaluation leaves the Network Interface and is processed by the Operating system. You would be increasing the processing load roughly by a factor of 10. I have seen flat networks with 400 - 500 nodes and documented a 300 - 400% improvement in overall network and system performance by implementing layer 3 (VLAN) segmentation. (Testing was based on Latency, sustained and burstable throughput measurements, and key application response times.
the next problem comes from stability in you Microsoft (I assume you have MS based nodes) Browse structure. You will have 10 times as many nodes to participate in each browse election for a subnet, as well as 10 time the chance of having an unwanted or uneeded Browser election. If you are thinking of having multiple subnets share a common broadcast domain (keep the addressing the way it is and get rid of the VLANS) you will instantly have severe Browse Structure problems. The MS Browse structure is based on a heirchy of NT Domain, Subnet, and System Level/function. The subnet portion is the key here. The devices want to elect a browse master for each domain/workgroup for each subnet. This is done with a broadcast. If the broadcast reaches nodes in another subnet (this would happen if you kept addressing and got rid of VLANS) you would start elections that could not be properly won. Access to your resources would be intermittant, Performance would be bad, and troubleshooting would be a nightmare.
I am running out of wind, so I will get off of my soap box, and just say "Don't Let Em Do It!"
let me know if you need more amo, or it this is not clear,
Jay Mosser
jaym@optymgroup.com