Boosting in essence is the practice of using a high level to run lower levels through instances at very fast speeds.

There are all kinds of guides, tips & tricks, suggestions on how to boost particular instances so please do a lil searching for that info.

Basically you want a class that has AOE capabilities. Yes a 70 can whoop just about any butt on preBC instances but if you have to target mobs individually it will slow you way down. For most of the lower level instances a mage with AE works very well. The reason most suggest a pally is for the plate+consecrate combo. You have plate armor to mitigate most dmg coming into you. You have a very nice AOE in the consecrate. Plus you have several "oh crap" abilities to save yourself in the event you bite off more than you can chew.

Instances are used instead of questing or grinding because of the particular problem you noted with "tagging". In the instance since the mobs are elite level the lowbies get a nice chunk of xp from the kills even tho the booster is so much higher level. Also since it is an instance you pretty much know the layout and the spawn points of most mobs, so its easier to plan and run in a timely manner.

So yes you will have to drop one team member to allow the high level in to boost with.

Most ppl have stated that boosting is faster than questing to level, however it can be godawful boring. Running the same instance over and over and over again until you either max out the xp for the instance or you hit a level where you can move on to the next set.

Hope this helps.