Istio w/o external load balancer

What are the disadvantages of using Istio without an external load balancer? There is none in my bare metal install and I am considering adding one but I believe Istio Ingress gateway does some load balancing already and so do the services. When is an external load balancer needed. Its just going to point to the Istio Ingress Gateway anyhow. Also is there just one instance of Istio Ingresss Gateway or can multiple gateways be run at the same time?