Originally Posted by Kane

IMHO the whole 'shock and confuse' tactic has been blown out of proportion. The P90X infomercial comes to mind . The way I think about it (and may be wrong lol) is in terms of movement and muscle contraction. If you take barbell rows and some sort of machine row, both have the same ROM and you're generating the same contraction with both...then what is the difference, to the muscle, between the two exercises? (Maybe those were bad examples, but hopefully you can see my thought process )

As far as exercise changing goes...It all depends on what you're doing and what stage you're at.

If you can keep progressing with the same exercise then why change? By the same token if you can't progress anymore or you need a break from that exercise, why not change?
Maybe you need to throw in some variations, its based highly on the individuals needs and that timeframe.