Why we're not currently interested in PXE-based Linux installs
In theory, burning Ubuntu install DVDs (or writing USB sticks) and then booting servers from them in order to do installs is an old-fashioned and unnecessary thing. One perfectly functional modern way is to PXE-boot your basic installer image, go through whatever questions your Ubuntu install process needs to ask, and then likely have the installer get more or less everything over the network from regular Ubuntu package repositories (or perhaps a local mirror). Assuming that it works, you might as well enable the Ubuntu update repositories as well as the basic ones, so that you get the latest versions of packages right from the start (which would deal with my wish for easily updated Ubuntu ISO images).
We don't do any sort of PXE or network installs, though, and we probably never will. There are a number of reasons for this. To start with, PXE network booting probably requires a certain amount of irritating extra setup work for each such machine to be installed, for example to add its Ethernet address to a DHCP server (which requires actually getting said Ethernet address). Ways around this are not particularly appealing, because they either require running an open DHCP server on our primary production network (where most of our servers go) or contriving an entire second 'install network' sandbox and assuming that most machines to be installed will have a second network port. It also requires us to run a TFTP server somewhere to maintain and serve up PXE images.
(This might be a bit different if we used DHCP for our servers, but we don't; all of our servers have static IPs.)
Next, I consider it a feature that you can do the initial install of a machine without needing to do much network traffic, because it means that we can install a bunch of machines in parallel at more or less full speed. All you need is a bunch of prepared media (and enough DVD readers, if we're using DVDs). As a purely pragmatic thing this also vastly speeds up my virtual machine installs, since my 'DVD' is actually an ISO image on relatively fast local disks. Even a local Ubuntu mirror doesn't fully help here unless we give it a 10G network connection and a beefy, fast disk system (and we're not going to do that).
(We actually have a local Ubuntu mirror that we do package upgrades and extra package installs from in the postinstall phase of our normal install process. I've seen some signs that it may be a chokepoint when several machines are going through their postinstall process at once, although I'd need to take measurements to be sure.)
Finally, I also consider it a feature that a server won't boot into the installer unless there is physical media plugged into it. Even with an installer that does nothing until you interact with it (and we definitely don't believe in fully automated netboot installs), there are plenty of ways for this to go wrong. All you need is for the machine to decide to prioritize PXE-booting higher than its local drives one day and whoops, your server is sitting dead in the installer until you can come by in person to fix that. On the flipside, having a dedicated 'install network' sandbox does deal with this problem; a machine can't PXE boot unless it's physically connected to that network, and you'd obviously disconnect machines after the install has finished.
(I'm going to assume that the Ubuntu network install process can deal with PXE-booting from one network port but configuring your real IP address on another one and then not configuring the PXE boot port at all in the installed system. This may be overly generous.)
The ultimate reason probably comes down to how often we install machines. If we were (re)installing lots of servers reasonably often, it might be worth dealing with all of these issues so that we didn't have to wrangle media (and DVD readers) all the time and we'd get a faster install overall under at least some circumstances. Our work in learning all about PXE booting, over the network Ubuntu installs, and so on, and building and maintaining the necessary infrastructure would have a real payoff. But on average we don't install machines all that often. Our server population is mostly static, with new arrivals being relatively rare and reinstalls of existing servers being uncommon. This raises the cost and hassles of a PXE netboot environment and very much reduces the payoff from setting one up.
(I was recently installing a bunch of machines, but that's a relatively rare occurrence.)