Itis NOT a con although the price many would agree is, big gulp, excessive.
Hygroscopy is real and if you read your definition of it again, which is correct BTW, you will realise that the fluid would would still need changing after a couple of years even if you did ZERO miles
The water absorbed into the brake fluid, over time
not miles, can overheat and then boil during heavy or constant braking, ie lots of downhill braking or very spirited riding . You will then suffer a very sudden loss in braking force , resulting in possible death if you are on 2 wheels.
I'm not a scientist/physicist , never went to uni, so really couldn't say when you need to change it 1yr, 2yr, 3yrs, 4 yrs
I guess the relative humidity where you live would have a bearing but hey ho better to be safe than sorry.
Absolutely spot on Mr Rushy.
If there is no after market warranty to protect do it yourself. EDIT. The following applies to bikes without ABS or servo systems. These will need extra stages and more knowledge.
At the most basic level you need a bottle of the correct fluid, read your owners manual (DOT 4 for 1150's), an old large coffee jar or similar not plastic, about 0.5 metre of clear neoprene tube (aquarium supplies), a few nuts that are tight fit on the tube and the correct single hex. spanner for the bleed nipples. You can buy these from car accessory shops or online. Get a good one that is an accurate fit.
Put about 25mm depth of fluid into the bottom of the jar, observing the wonderful pale colour of the pristine product.
Force the nuts onto one end of the tube and lower it into the fluid. The nuts will keep it on the the bottom of the jar. This will stop air being drawn back into the caliper.
Ensure that you can ease the bleed nipple open. Old ones that have not been shifted regularly will be seized and will sheer without a lot of force. DAMHIK. (THis is not an immediate disaster as the caliper will still be sealed and usable. You can even still bleed it by releasing the banjo but it is messy and fiddly.)
Place the other end of the tube on the nipple with the spanner already located.
For the front align the handlebars to get the reservoir as horizontal as possible. If you can lock them in place all the better.
Cover all painted surfaces close to the reservoir with plenty of paper or rag. The fluid attacks the paint. If it does splash on the paint as long as you notice and clean it off immediately it won't cause a problem. On 1150's if you let the fluid level drop too low it shoots a jet of fluid out of the master cylinder as you squeeze the lever. Again, DAMHIK
Open the reservoir and ensure that the fluid is topped up to the maximum mark.
Ease open the nipple and press the brake lever. Fluid will enter the tube from the nipple. (Observe the dirty brown colour caused by the water absoption and possible onset of corrosion. It is alright to feel smug at this point.) Bubbles of air will appear in the jar fluid.
Pump the brake slowly whilst ensuring there is always plent of fluid in the reservoir until the fluid in the tube is the new clean variety. Then holding the brake lever in the "on" position close the nipple. It only needs one finger on the spanner, a ladylike one will be adequate, a big tosser fist will break the nipple. In fact just remember you are dealing with a nipple, gentle is good.
Carefully remove tube from nipple. One caliper is done. Always do the furthest from the reservoir first, so the left front disk on a bike.
It takes less time to do than it has taken me to type using two thumbs and eight fingers.
Always ensure everything you use it spotlessly clean. Keep the jar complete with some of the fluid (and with the lid on) and the tube for the next time. Put the tube in a grip top poly bag to keep it clean.
Spend balance of £170 on a UKGSER weekend event which will be much more fun than giving it to a dealer.
P.S. Brake fluid isn't very nice stuff unless you are a brake system. You shouldn't drink it or use it as eye wash.
I thought it was the same with the new bike standard warranty 
