This is a great thread. The quality of posts is really much higher when the topic is training instead of pro running (or certainly "other" topics).
There are pros and cons to everything, and a well-executed 6-8 mile run at ~40 seconds slower than 5k pace is an excellent workout. It's just very hard to execute right. A suitably intelligent/patient/well-trained runner might be able to consistently execute their tempo runs, but a lot of things can go wrong:
1) Run a pace that feels good, but you realize by the end it was too fast
2) Feel "off" at the beginning, and never quite get past that feeling
3) Start too fast/slow, under/overcorrect when you try to adjust pace, and never settle in
4) Pick up a twinge, and not know whether you should stop or not
I personally enjoyed tempo runs, but have always struggled on them. I've fallen into each of the failure modes above, multiple times. Meanwhile, intervals at threshhold are a very similar workout that would almost always go smoothly.
It's a little hard to explain why tempo runs are so difficult, but one reason might be that you're trying to hold a precise pace at an effort where you don't have a lot of sensitivity. At closer to race pace, try speeding up by 10s/mile, and you'll feel it right away. On easy runs, pace differences that small barely matter. Tempo runs are this weird middle ground where you do want to lock in the pace, but it doesn't feel (to me) like a natural equillibrium. If pacing in a race is like running on a knife's edge, a tempo run is like playing that game "desert bus".
Another way to put it is that you're doing "race-like" things: trying to lock in your pace, running continuously for several miles, but your adrenaline level is (or should be) barely higher than an easy run.
There are certain cases where tempo runs might be essential (half marathon or marathon training, perhaps a newer runner who struggles to hold any pace). Or maybe you're actually good at them, in which case bully for you! But there's something to be said for doing workouts that you can reliably execute well. For me, that has never been tempo runs (much as I enjoy them).
Maybe I'm just weird on this, but it seems from the rest of the posts that many of you have had similar experiences.