Having a large amount of experience with backpacking using both bottles and bladders, let me underscore and agree, to an extent, what t2andreo has pointed out about bladders. They can be a harbor to bacteria and molds that will affect water quality. The caveat is to define under what conditions such would occur.
When a reservoir is used on a daily basis and is being refilled and depleted of its water as it is used, it is no more susceptible to problems than a water bottle. At the end of the day, it is a simple matter to rinse out a bladder and either refill it and put it back into its spot in one's pack, or to keep it open and let it dry out as you would a water bottle. The opening on my Platypus Big Zip is simply propped open slightly with piece of a chopstick and hung upright. It dries out quickly this way.
The real problem occurs with storing a reservoir that won't be used for a while. When doing so, it is simple to rinse out, fill with water, add about 1/8th of a cup of household bleach, and then proceed to drain the bladder via the mouth tube. Set it aside to dry, OR, you can stash it in the freezer until you next need it.
If while on Camino, you wish to do more than a rinse and dry and periodically wish to do a formal sanitization, then there are inexpensive tablets one can purchase for this purpose at any outdoor store in Spain. Or even the equivalent of an Effedent tablet for dentures. Simple fill the reservoir, drop in a tablet, and let it sit for 30 minutes after running a bit of the water thru the tube. There is no need to carry even the minimal weight of these tablets since you can pick these up easily enough. This works well for bottle users, too.
If bleach is available at an alburgue, you can use a bit of that in a filled bladder instead of tablets.
Another method of sanitizing is to use the UV radiation of the sun. Fill the bladder with water and hang it exposed to the sun on the back of your pack or on a clothesline.
As I said, though, there is no real need to disinfect when the bladder is in daily use. The exception would be, just as with a bottle, if you accidentally put water from a potentially contaminated source into the bladder.
The weight issue was also rightly addressed by t2andreo. My 2 liter reservoir weighs in at about 4 ounces. A typical empty water bottle, like the 1 liter sized Smartwater bottle that is popular with backpackers, is about 1.3 ounces in weight. So the equivalent 2 liter weight would be 4 ounces for the reservoir vs 2.6 ounces for the bottles. ( 113 grams vs 74 grams).
