My reply with the help of the TFP moderator and a professional chemist:
Boric acid is not used as a filler, but as an active ingredient. As a general rule of thumb, a combination of chlorine with boric acid is almost always going to be more effective than chlorine alone -- it doesn't matter if it's dichlor or trichlor, especially in inhibiting algae. Boric acid and chlorine both attack bacteria in different ways; and with a combination you're going to get a 2-prong attack -- with softer water and less irritation to skin and eyes being side bonuses.
Boric acid solutions are used in eye washes, as an antiseptic or to treat some infections, as it is anti-bacterial and inhibits growth of microorganisms. Some industries use it as an insecticide.
http://en.wikipedia.org/wiki/Boric_acid
http://www.natbat.com/What Is Boric Acid.pdf
However, the use of boric acid does not replace the need for an effective sanitiser (chlorine). Bacteria, viruses, and other microorganisms still require quick kill times to keep a pool safe. When using borates in the pool, consider who will be using the pool. The small amounts of water inadvertently consumed during normal pool activities do not pose a hazard, but pets or small children may be inclined to drink large quantities of pool water. Consuming large amounts of borate-treated water is toxic, as is drinking water with excessive borate levels.
When using liquid chlorine, be very careful of accidental spillages. Apart from being corrosive, the fumes from spillage can make you very sick. I know of several cases where commercial pool workers have had to be hospitalised due to accidental chlorine spillages.
Liquid chlorine also loses its potency over time. After about 30 days, using typical storage methods, you can expect a noticeable reduction in concentration. It starts to weaken as soon as it leaves the manufacturing facility (could be local, interstate or overseas by road, air (unlikely) or ship), then the warehouse to await an order before despatch, then the retailer's storage area, then the retailer's Point-of-Sale area before it reaches your home. This is not even counting the amount of time the liquid chlorine may sit in retailer's storage because they bought the item in bulk (for a discount) in anticipation of the upcoming season or due to a winter sale. Quite often, the strength listed on the label is not its true strength, so always test the water before adding.
Although granular chlorine does not lose its strength like liquid chlorine does, you can inhale dust while broadcasting into the pool on breezy days, also making you quite sick. Because granular chlorine is potent, also be careful during application.
Cal Hypo is highly reactive and explosive when not used properly, mixed with other chemicals or stored properly. I have personally seen how quickly Cal Hypo can combust when accidentally mixed with other chemicals in 2 incidents (i.e. you may not have time to get out). The mixing with other chemicals usually occur during accidental spillage in storage.
Leslie's Pool Supply in Dallas recently exploded and went up in flames because of Cal Hypo. $6.2 million of inventory destroyed in minutes:
http://crimeblog.dallasnews.com/archives/2011/08/atf-investigators-brought-in-o.html