Doesn't necessarily need to be a new position, merely an actual possibility. After all, improved sight range would allow you to station a dwarf up a tower and expect him to see nasties coming. However, from a programming standpoint, improved sight range introduces a whole host of performance issues. If every dwarf has the ability to see x squares out then you have to process the contents in those squares, if not every frame, probably every ten at least. That's a significant number of cycles considering the population in an embark area. If there was to be an actual task (not even a position, just a command to stand watch), you could cut down the number of units that you have to compute sight range for to a handful. It'd be a trivial addition (there are at least three different ways I can think of to do it off hand, all of them involve logic models already present in DF) and I think it'd add something to gameplay that isn't already there: The ability to actively protect your fort from invaders by trying to remove the ability that allows them to do the most damage (at the moment, improved sieges will probably make ambushes seem trivial, instead of the other way around).
-D.B.