But the important point is this I think: Beethoven still regarded (as did his Classical predecessors) the Trumpets as being used mostly in conjunction with - and usually still merely doubling - the Timps.
I don't know if there's just a single important point here
- another problem with filling out the trumpet line is that it puts a climax where Beethoven didn't write one (it's still only forte there) and the build-up after that to the real fortissimo becomes a bit redundant. If he'd wanted to write a big thumping unison line he certainly could have but in fact at that moment the winds are chattering away in a repeated-note version of the arpeggio theme, the violins and violas are scrubbing away in long tremolos and the basses are scurrying up and down the scale.
(Did he ever actually write a top concert Bb for trumpets, by the way? He certainly went up to the written G for the D trumpet but I've forgotten if he ever wrote notes sounding higher than that.)
Back in my school days I used to pencil in similar little 'improvements' for brasses and timps and winds but sort of lost interest when I realised that the process could never really stop, especially if you consider the possibility of not only 'correcting' the places he used the instruments but also using them where he didn't use them at all.
Also that the orchestrational limits are to some extent part of the composition - or at least that they form part of the drama of the piece as played, whether Beethoven planned them that way or not. For example, in the first movement of the ninth symphony there's a dotted figure with timps which is then jacked up a notch to where the timps can't take part (bars 102-3 and 106-7) - pedal timpani could play the whole figure but as it is the second bar of each figure has the ground cut out from under it by the disappearance of the drums. Maybe he would have been happy to have the drums each time; maybe the idea was precisely to emphasise the general sense of developmental drift. And even more obviously in the Scherzo of the ninth - the timpani are tuned to F and f, there's no timp on the tonic although a third drum wouldn't have been unprecedented. This means the timpani insist on the minor mode even though everyone else keeps trying to escape it - the crassest example is bar 338 where the orchestra is burbling along happily in D major and it's the timpani that turn everything back to the minor. (martle, I don't quite agree that Beethoven would write actual wrong notes for the timpani just because they can't project pitch so clearly - as far as I know the notes he wrote are always the notes he wants. Some later composers did indeed do that though.)
Certainly it's part of the package of Classical and Baroque orchestral music that modulation brings a change of orchestration with it; if you change that aspect I think you're subtly undermining the way form is projected through the instrumentation.
Especially on a warm summer evening.