Interestingly enough Dave, you just described three of the four basic foot movements we do in training at the beginnning of every class.
These are not exactly ma ai exercises that we do, but a structured way of learning basic movements of evasion and entering to obtain a particular position for technique while staying relatively safe.
A ma-ai exercise that we do is called tegatana awase, which involves maintaining the distance of issoku itto (one step to strike) by maintaining the unbendable arm distance with our partner and responding to subtle unplanned movements of entry/retreat/side to side by him/her.
I am not one for the concept of "you should never step backwards" however. As it can often be a good tactic to re-establish the distance you want to work at if someone is closing on you too quickly or unexpectedly (e.g. a grappler). I do believe that we should not step back constantly, which will create the cornering/severe loss of ground situation described earlier.
I have often used stepping backwards or alluding to stepping backwards as a way of making uke overextend the attack, helping me to shoot in for technique (like shomen ate), with uke actually helping me to close distance by his overextension. It works the opposite way too.
Just my 2 cents.