leia in lalaland,
I'm not aware of any new pool construction where main drains are eliminated. I have eliminated drains in old pools when they are leaking, such as copper plumbing that was used years ago. Pool cleaners that pick up debris don't need drains, but they do provide a function in the recirculation system.
I split the drains on all older pools whenever I do a remodel. It's preferable to bring it up to code even if you don't get a permit for the remodel.

Including a main drain in a quote is simply a new cover. I see no logical reason to eliminate a vital part of a pool that has been around for, eternity.

repair_guy, do you mean the "main drain" in a quote is just the metal cover for the drain?

Our pool was apparently built in 1961 (along with the house) and the copper plumbing has stained the old plaster. We're having all new plumbing installed.

Replacing the main drain in an existing pool is expensive and a total PITA.
I also always recommend it to my pool reno clients.
The options are to either leave the old main drain and pipe in place, asking for a leak, or to eliminate the main drain, limiting the circulation in the pool and causing potential future complaints of the deep end being cold.
I always discuss this fully with any customer.
As golfgeek said earlier, the least I will do if the client does not want to replace the fixture is run a separate main drain line from where it connected to the old skimmer back to the pump.

I really dont see a purpose in having any main drains if you have a suction side cleaner in a pool. In theory wouldnt that be like having a main drain drawing water from the bottom of the pool all over the place? Why would you need a main drain then? Cleaners get stuck on them some of the time too!!!

The main drain serves a purpose. When attached through the skimmer as all skimmers are designed, it is an equalization line. Should your water level drop below the waterline, it sucks from the main drain. It is not a means of draining the pool as the names states.

True, older copper main drain lines leak and in this case it is best to eliminate them if you don't want the cost of replacement, but at that point, a wall equalizer line should be put in which is a simple hole if you are doing a rehab.

Most plaster jobs will include new "main drains" in the bid. That means the existing cover and ring will be wiped out in demo and a new plastic anti vortex cover with ring will be plastered in assuming the line is good.

With auto fills being used so much in new constuction and added in a remodel the worry for the waterline dropping below the skimmer is eliminated. In this case where is the main drain still needed?

I couldn't imagine not having a main drain - that's where I brush the dirt from the bottom of the pool to and send it through the filter. :)