Authentic Instruments?

I just heard a splendid live performance of Mozart's Serenade for Winds. All the musicians, including the two who were playing the basset horn parts, were playing modern symphonic instruments, not the instruments Mozart wrote for in the late eighteenth century. 

I'm not complaining. It sounded wonderful. However, if the argument for playing baroque music on baroque instruments is strong, why isn't a similar argument, equally strong, made for playing Mozart, Haydn, and Beethoven, among others, on the instruments of their times (which were not essentially different from the instruments of the mid-eighteenth century)?

A few years ago at a music festival in Warsaw I heard a symphony orchestra that used old-style instruments and arranged the musicians the way they used to be arranged in the nineteenth century, and I was impressed. Obviously if a symphony orchestra today is playing a program with music from the eighteenth through the twenty-first century, it has to use modern instruments (most of which haven't changed all that much since the Industrial Revolution), and, within limits, it can't keep rearranging the players. So we hear what the conductor has in mind and places on the stage.

Incidentally, the Mozart Serenade calls for two basset horns, which are similar in range to modern alto clarinets. Authentic basset horns aren't exactly common, so I assume the two musicians who were playing in the Serenade last night were playing alto clarinets. If a clarinettist wants to own a range of professional level instruments - a B flat and A clarinet, an alto clarinet, and a bass clarinet - he or she has to invest a considerable amount of money, which, in general, professional musicians don't have.