Simple picture analysis will show exactly what i mean:
some of top 10 men:
http://www.theage.com.au/ffximage/2007/06/11/nadal_forehand_wideweb__470x290,0.jpg
http://z.about.com/d/tennis/1/0/h/C/federer-forehand-inside-out.jpg
http://www.bloomberg.com/apps/data?pid=avimage&iid=iQrhp2qVbyUU
http://farm3.static.flickr.com/2154/2406725301_ec8d83492a.jpg
http://farm3.static.flickr.com/2183/2313951819_876c998304.jpg
http://www.tomveatch.com/tt/images/Roddick.Vgrip.jpg
http://cache.gettyimages.com/xc/77378535.jpg?v=1&c=ViewImages&k=2&d=17A4AD9FDB9CF1938DCDF9EF37AEBD7374974F60E534D249284831B75F48EF45

fed,nadal, verdasco, and to an extent nalbandian all have almost straight elbows during hit. This allows for a more fluid hit, less strain on your shoulder, and more reach - among other things.

some women:
http://blogs.abc.net.au/photos/tennis_australian_open/anna_forehand_2.jpg
http://www.lta.org.uk/PhotoGallery/The%20Championships,%20Wimbledon/The%20Championships,%20Wimbledon%202007/Venus%20Williams%20forehand%20-%20Photo%20by%20Tommy%20Hindley.jpg
http://cache.gettyimages.com/xc/564212.jpg?v=1&c=ViewImages&k=2&d=17A4AD9FDB9CF19390335F8FA9CA92A688710A2782CB5794A40A659CEC4C8CB6

why do you think sharapova is having shoulder issues - because if ur elbow is bent like that, and you accelerate with your body, your rotary cuff (which is what will take the hit) will not be able to stand the sudden force at the end of your stroke.

I can explain using physics how it is better to have a stroke which ends up as extended as possible, but just take my word for it.

So my question, is if everyone in the world sees how the best players (and players who are among the best, or soon to be among the very best like verdasco), have an extended forehand, why do they continue hitting the ball the way they do?

[also, hitting, ending with a straight elbow, means that you are getting most of the force transfered through your pectoral muscle (which has a much wider range of motion than the rotary cuff)]

I'm saying that people should be taught differently - but i constantly see tennis-channel academy vids with juniors learning to hit with their elbows bent

No it doesn't give u more top spin with western grip - look at rafa...western grip, straight arm, more spin than anyone can handle ...quite literally