Well I see your points, but if these methods are available, why not using them? Personally I don't find anyhting lazy about it. Beside even calculating it by yourself doesn't require any complexe math formulars... Unless your in the French aviation of course which are known for their particular method of calculating things in a more complex way than their anglo-saxon neighboors!
Well, it could save your @ss one day if you need to get down in a hurry (fire), and besides that, the VNAV deviation is only 'valid' if you stay on the FMS centerline.
It's during radar vectors is where you see most people screw up their descent planning, because suddenly they have to use their brain and guesstimate the amount of track miles and number in sequence, how much you need for configuring at that particular LW, and also factor in tail/headwinds on final.
p.s. my experience is that most Brits were much much more of the 'just-do-as-your-told-and-follow-the-magenta-VNAV-line' persuasion, than for example the Spanish, Germans, French, Belgians or Dutch. Dumbing down aviation seems to be the name of the game these days.
The biggest nervous flyer was some hot shot Aussie who at 10nm and 3000' HAT would start with "I think ya need the gear mayte." My funniest experience was with an ex-Big Airlines captain who, from FL390 on, started pulling the speed brake each time VNAV (or whatever it was called on the iBus) was showing ±200' high. (we kept on getting intermediate level offs because of traffic just below us) He was a super nice guy but just for fun I started counting the number of times he pulled the speed brake during the entire descent, but I think I lost count somewhere after 20.
The concept of being able to 'dive off the altitude' by simply selecting a slightly higher speed, or the fact that we might even get a few more track miles when being radar vectored by approach, seemed to be completely foreign to him.
