Not a necessity, no. It looks like I was just speaking for myself. If I had lyrics, I would consider them pointless unless they could be understood.

If someone were to write lyrics with the intention of making them understandable, then I think that would be a limitation.

This is correct. In listening to pop music we've gotten used to hearing vocals as the most dynamic part of songs and a repetitive riff, hook, or other simple melody on the background instruments. But indeed, this doesn't have to be the case. Operas generally have complex instrumental arrangements to go with the vocals. Take a look at Wagner, for example.

There's also the fact that for example in rock the singer is not singing the entire song. And even if they are, the songs can still be complex. Think he's already been brought up in here, but look at Frank Zappa. He was certainly making complex music, but there's lyrics in his songs. Or you could look at, for example, scat. The voice is used as a primary instrument rather than to deliver lyrics. I think there should definitely be a distinction made here between a song with vocals, and a song with lyrics. Because your voice is really just another available instrument.

Why would you consider that pointless?

If I were to use words, I would care more about the meaning of the words than I would their melodic qualities.

I don't know why everyone feels the need to mention jazz or prog to justify any other type of modern music.

The main riff to 1969 by The Stooges or the bassline to Waiting For An Alibi by Thin Lizzy just to name two examples are just as much a thing of beauty as anything any classical composer has ever composed.

If you don't think it is then it's just snobbery in my opinion.

The simple things in music are often the most effective and the most beautiful.
This goes for all kinds of music. Classical included.
In fact, it's the musical gymnastics that usually fail a good tune no matter what the genre.

What about Fur Elise and Sugar Plum Fairy? Those're two classical music pieces that I like. Would they be considered as "complex" as normal classical music?

Complexity in music is pointless unless there's something more behind it, though that's just my opinion. There are of course many who revel in overzealous technicality, but perhaps they just looking for something else in music besides emotional or lyrical satisfaction, such as stress relief, something to bang one's head to, etc.

And Urban, on a side note here -- people who look down on progressive rock, jazz, etc. and write it off as overwrought weirdness are just as "snobby" as the people whom they consider snobs. Folks tend to forget the fact that when albums within those genres are done right, the results may often be quite convoluted, but are nevertheless beautiful and possess just as much meaning from a lyrical perspective as anything The Fall or whatnot have recorded.

why place an arbitrary limit on the original instrument?