Not running VLANs results in security/stability drawbacks.
Many clueless and/or smaller operations put everything on one Ethernet segment. Unfortunately, this makes it very easy to break the entire network by accident or malicious intent -- bind the IP address of a different machine on the same segment, and it's unreachable. (What if that's a DNS server or router?) ARP races are not your friend. It's also a security risk to have many machines on the same Ethernet segment. If one is cracked, it's very common for the perpetrator to install "sniffing" software to catch passwords.