Stu, With Drop D you always have that drone on the low D string(really makes things rumble,same applies to DADGAD)and low G when you capo 5 and it faciitates playing chords up the neck and also ascending and descending bass runs that might be difficult in Standard tuning. Key to drop D is learning to play a 2nd position D chord which is basically a first position C shape with the pinky playing the D at the 5th string,5th fret and the index finger playing the A at 2nd fret 3rd string. Lots of possibilites in that position for bass runs alternate, modal and moving chords.   
That said,there are tunes that work better in Standard so I put a Hip Shot tuner on the Low E string of my guitar so I can quickly change between standard and D, some times in the same set. I guess I use Standard and Drop D about equally and use DADGAD for fingerstyle.

Of all the tunings I think standard is probably the most versatile, at least in the first position. Just my take.