Transmissions are all over the map now for expectations and maintenance. For most passenger cars they really can 'go for life' on the factory fluid unless you're really running the car past the 300,000-400,000 line which most don't make. Heavier duty vehicles like trucks and vehicles that are built for towing you will notice that they do call for transmission service on a scheduled basis since the fluid is under much higher demand and wear, but even these are commonly neglected and will go long without major issues.
Flush vs drain & fill is the big issue. Transmissions tend to pickup little bits of gunk and varnish here and there in the passages and valve bodies as they age, and heat cycles, and particularly overheating, ie towing a trailer larger than what the vehicle was designed for, or spec'd for (no transmission cooler/tow package) will cause rapid varnish buildup. There's an argument to be made (and I've seen it happen, so it's not just blowing smoke) that a long neglected transmission be left neglected, or if you're going to service it anyways do only multiple partial changes of fluid to slowly get some of the properties of newer fluid back without disturbing too much internally - What happens on long neglected trannies where a full change (or worse yet, a "flush") is performed is all the disturbance of the flush and new fluid causes all those little bits of crud and varnish to come loose and then promptly get stuck in places where they cause all sorts of grief.
So, it's really one of those things where you either maintain then routinely, if if they've been neglected, it's almost better to leave them that way - you could just be causing yourself issues.
