I think, with the decrease of LCD screen prices, it's becoming more and more common. However, it really depends on personal preference. If you use your notebook on the road and then as a desktop replacement, you might be able to justify the extra expense. Otherwise, I'd suggest just trying to live with the notebook's display. Do you use your notebook as a desktop replacement? Do you find the screen too small at times? If you answered 'yes' to both of those questions, you're probably a prime candidate for a new 30" LCD display. 😉 (Or whatever size suits your needs.)
Squire