I take it you mean isometrics as in, things like planks or push up holds?
I think they are a good tool and in some situations can be extremely valuable. I also think the 'core' should be trained primarily in an isometric fashion. That's my opinion so, I'm not into situps and crunches.
You just need to be carefull and not over react to somethign which you have found to be useful and end up devoting most of your training to it - never stray from the basics unless you have a very good reason, they should always be the core of your routine.
I do all my reps with a one count at the hard part of the repitition. Was something I was told to do in physical therapy and carried it over to all my lifts. Makes the lifts "harder" in that I fail with less reps, although I'm not sure the time efficiency versus just pumping fast.
I'm not sure what the reasoning would be behind that. Did they tell you? Making the lift harder isn't much of a reason. In general, you'll benefit more from a controlled eccentric and an 'explosive' concentric (the hard part of the lift). There are times when it would be beneficial to slow down the tempo and other things like that but, that's an exception other than the norm although it's good to use a slower tempo on a new exercise until you've sussed the movement. Then you would just lift fast.
That's my views, anyway...