There is just as much chance that they will NOT be impeached for actions that occur before they took office, so your point is moot.
In fact, during times when impeachment WAS considered for previsou so-called "crimes," they committees decided they could NOT use those as evidence for impeachment, as I already referenced concerning Richard Nixon.
The prior crime would need to be weighed against the damage it's done to the officeholder's ability to continue in his current capacity. Also, the amount of damage done to the integrity of the office would be weighed.
So, IMO, a serious enough offense would warrant impeachment. Just because no impeachment has occurred to this point doesn't mean it isn't a remedy available in the future. (Please note, it doesn't appear there's any evidence at all against VP Cheney. I'm speaking about hypotheticals.)
No, your original point is moot. You had stated before that a public official could not be impeached for actions taken prior to their current public office, when they clearly can.
I never stated that this happened a lot, simply that it could.