Mark Ward addresses the issue here. I would add two points:
(1) It seems to me that the excellence and beauty of the poetry in classic hymnody on average exceeds that of newer hymns. This is doubtless true, at least in part, because over time the very best in poetic expression has risen to the top and less stellar contributions have faded away. At the same time, I do think Christians in time past were generally better able to handle language, and poetry in particular, than they are today (in part, I suspect, because their surrounding culture, as a whole, could). The more that classic hymnody is “updated,” the more likely it is that the original excellence of a given piece is going to be compromised to some degree. I don’t disagree with the idea of cautious and limited updating for some reasons (here is my parade example), but in my thinking, such reasons are very few.
(2) Hymntexts that sound like they were not written in our own day, but are from time past, help to highlight the continuity that the church has through time. Scripture does emphasize the “new song,” and believers ought to be producing new songs. But singing things that were clearly not written in our era, we remind ourselves of our historical rootedness, and the faithful witnesses who have gone before.
Jubilate Deo 