Seriously though; you make a good point, but I think it differs person to person. I've often found myself coming up with storylines and gameplay based exactly on what I end up creating with terrain (both in SC2 as well as other games). I rarely work on a more solid plan than "this terrain should consist of an X on planet Y, featuring perhaps some Z". Some folks plan and work when it comes to this, some work and plan as they go along. I don't think one way is necessarily better than the other, though guarding for a playable map is nevertheless a good thing to be reminded of.
I can't condone this advice :P
Seriously though; you make a good point, but I think it differs person to person. I've often found myself coming up with storylines and gameplay based exactly on what I end up creating with terrain (both in SC2 as well as other games). I rarely work on a more solid plan than "this terrain should consist of an X on planet Y, featuring perhaps some Z". Some folks plan and work when it comes to this, some work and plan as they go along. I don't think one way is necessarily better than the other, though guarding for a playable map is nevertheless a good thing to be reminded of.