Check out this article: http://www.lrc.rpi.edu/programs/nlpi.../lat5/pc1a.asp
The average life span of most T5s and T8s is 20,000 hours. That's like 5 years. Even after 40 percent of their life span if used up (22 months) T5s still have 94% output. So if you're changing bulbs every year, or even worse, every 8 months, your are spending a great deal of money to regain a couple percentage points.
The article is ten years old, but I think that makes for an even stronger case as bulb quality and efficiency has only improved over the years.