Once again the global nature of the PPRuNe audience is generating red herirngs... and any debate becomes tricky once you start to consider different nationalities modus operandii... and any difference there maybe therein between Mil and Civil procedures (and believe me there are MANY differences)
In the UK the Mil are advised to use "Runway Track" NOT "Straight Ahead", or "Runway Heading"... so Paradise Lost, I'm afraid you're not quite correct(see the end of the next paragraph for the reason why)
I agree with Spitoon, to a point, and would suggest pilots fly the clearance... if told to maintain runway track do so, unless unable to comply (in which case tell us) and if in doubt request clarification. But as for wind drift not having ANY effect... I can quote the case of an Airprox that was filed at an airfield where one pilot climbed out on runway hdg... not on runway track (i.e. maintaining over the centreline) and came too close to comfort to another in the Visual Circuit
Finally, I tend to agree witht he comment that the instruction will invariably be given as a short term control. For example, I have traffic close to the SID track, moving away... I suspect that by the time the next departure is airborne and turning onto track the subject confliction will be clear, but to make sure I'd give a "runway track" instruction... then as soon as I can see clearly that seperation exists, I'd give "own navigation".... often this is the first instruction after take-off. In otherwords it's about avoiding delays, expedition and efficiency?
As for terrain clearance... at the places I've worked mountains haven't been an issue, so I cannot comment first-hand but would like to think that a Controller wouldn't use this technique if the SID included a turn simply for terrain?