If you keg there is really no need to use a second carboy as a secondary. The keg serves as a far better secondary (or bright tank) because you can purge it completely with Co2. You can then either blow the yeast out through the dip tube or, if you want to be sure the beer is completely clear so that the sediment doesn't stir up when you move the beer, rack the beer to another keg via a jumper. A shortened dip tube on the secondary or bright tank keg can help you leave the yeast behind.
If you bottle I can maybe see why a carboy secondary can be useful because you can leave a lot of sediment behind, but be aware that by using a secondary and bottling you will be racking the beer up to 3 times (ones to the secondary, once to the bottling bucket and then once to the bottles) so you will most definitely be picking up some oxygen and potential contaminants.
OTOH as others have mentioned you can get the beer really bright by cold crashing on the yeast cake and racking directly from primary to bottling bucket. But there are a good share of homebrewers who don't have the cold storage space to cold crash 5+ gallons of beer.
As far as yeast stripping the hop oils and aroma, it is best to dry hop once the yeast has flocculated. But you can always just add more yeast or crash the yeast in the primary first and then add the dry hops. I don;t understand why the yeast cake would affect hop oils or resins. My understanding is it is the yeast floccing out that drags these oils out of suspension.