Max native resolution is the key issue when it comes to scaling video sources. For PC games or OS desktop, it's not an issue, because they can scale to most resolutions and aspect ratios.
But aspect ratio is important when it comes to viewing non native scalable (can't alter aspect ratio without cropping or distorting image proportions) video sources like DVDs or BDs, many of which are set at 16:9, meaning you get black bars at the top and bottom of a 16:10 ratio display, effectively decreasing the area of the picture. So 16:10 aspect ratio monitors like 1920x1200 are not optimal for viewing movies, hence the availability of 16:9 aspect ratio monitors.
It's a bit pointless to rant about "missing pixels" since there are 16:9 aspect ratio monitors that are roughly equivalent in diameter to most 1920x1200 displays with roughly the same number of pixels like the Dell SP2309W which is 23" 16:9 aspect ratio 2048x1152 resolution (slightly more pixels than 1920x1200).