I do concentrated worts and add cold water after the aromatic hops to bring it to about 90 degrees or so. I've never done a whole 5 gal boil before. Is there a flavor advantage or anything else I'm missing?
there are several advantages, the first being sanitation, if you boil your whole volume you are assured of killing off any unwanted bacteria, but in the same breath I have never had a bad batch when using the cold water method. It depends on your source of cold water, tap water has chlorine not good, well water isn't sterile enough, bottled water is probably your best bet.
The second reason is actually much more important in respect to actual beer production. when you use a concentrated wort, you suffer weak hop utilization. your wort is too concentrated to efficiently extract the hop oils and flavors from your hops, so you must over compensate and use much more to do the same job. as your concentrated wort density varies from batch to batch you often get unpredictable bittering results which can be frustrating.
The third is a matter of the quality of the boil and how it affect your final result. I brewed on the stove for a few years and after getting a 100,000 BTU burner, I quickly saw the difference. A good hard rolling boil was just not achievable on the stove. the big burner made all the difference in actually creating a rolling boil and facilitating a hot break. foam will start to rise as you start to boil, proteins in the wort coagulate due to the rolling action of the boil. The wort will continue to foam until the protein clumps get heavy enough to sink back into the pot. once the foam settles down You will see particles floating around in the wort, This is the Hot break. coagulating the proteins in your wort and adding additional Fining agents such as Irish Moss, remove positively charged proteins in the wort, and give your beer a professional clean clear appearance instead of just another cloudy homebrew.