That's exactly how both the series I mentioned work! In addition, they both allow the importation and exportation of grayscale height maps (which can easily be made in Terragen, though I prefer Photoshop). After generating your heightmap, you just apply a color overlay of the primary color of your terrain, set your water level, then "paint" the detail in. Not a "one step" process, but pretty simple for most folks to master. Of course you could compose the entire map within the map editor (which used a wire mesh system for keeping tract of the height), but it's a rather slow, tedious, and error prone process that yields poor results. Height editing within the (MechWarrior and MechCommander series) editors is best used only for cleaning up details.
As for the propensity for low quality maps to rapidly multiply, this happens with every game, of course the same can be said for the quality of players one might meet online. For every good map out there, there are easily 100 or more bad ones. This is OK though, because it is hardwired in human nature to dismiss what is bad, and gravitate towards the good. Those maps that are good will become more popular, which in turn encourages their designer's to produce even more good quality maps, bad map designers will be shunned and fade into obscurity (the same way a true free market economy works)!
Anyone who doubts this can just look at ID Software and UbiSoft for inspiration. Two of the employees on the Doom III team started as fans of Doom II who created extremely popular levels that they posted on ID's site. Three of the current members of the Rainbow Six design team are also former fans who also were noticed by UbiSoft for producing superior game levels.
Point is, any yahoo can draw a map, but it takes a real artist (in the programming sense) to produce a challenging scenario.