The idea of using string reels for stringing rackets is not new, however there are a few things to keep in mind when using this approach.
Not all string reels are created equal. When comparing reels and sets remember that reels come in sizes ranging from reels that have several sets to 100ft,100m,300ft,600, 660 or 720ft. It can become confusing. A standard reel that is 660ft should be enough for 16 rackets (40feet x 16 rackets = 640ft). Each set of string is typically around 40ft (give or take 2 feet). THe reels generally will be cheaper to purchase than it would cost to buy the sets individually but in some cases may be the same. For example currently at online tennis stores you can either purchase Wilson Sensation for 9.95 per set or purchase a reel of 660ft for 129.95. In the reel you are basically paying for 13 sets and getting 3.5 sets free, this nets down your cost. Another example is to purchase a reel of 660 ft Wilson Extreme for $48.45 or each set is $3.50. You are basically paying for 14 sets. So you can see it will net down your price considerably.
Buying reels is also more environmentally friendly because you don't have as much package waste.
Reels are easier to store and creating hybrids is a snap because you can easily cut a 20ft section for half your hybrid.
It is easy to waste string when you are removing it off of a reel. I sometimes tend to measure a piece too long because I feel I have a whole reel so why bother.... One method that I have used that works is to find a place on your machine or work area and measure out a 2 feet section and mark it. Take your string and run it along your 2 feet mark and measure a set or half set using this method, it will help you from wasting string.
Some higher end stringing machines will have a place on the stand to attach reels and they have a measuring devise that you can use to easily measure the correct string amount.
Generally what I will do is keep a set or two on the string wall for customers to see and then I would use the string from the reel. This saves money, is great for hybrids and the customers can still see what the packaging looks like.
At tournaments most players will drop off a reel with the players name on it and the stringer will keep it throughout the tournament and return the reel to the player once they are out of the draw.
Most strings are available in reels except Natural Gut and most multifilaments. I have seen Wilson Sensation in reels but NXT or other premium multifilaments are not available mostly due to the drying out of strings. Individual sets are sealed but reels are subject to the weather elements. Polyester strings are popular as reels.
I hope this clarified some issues with reels vs. individual sets. Just be sure you know how many feet are in the reel because not all reels are created equal.