I've found the best thing for removing dry rot (or other rot) is a preasure cleaner.
It will rip out the bad wood and leave the sound stuff behind.

I'm in the process of doing my own facias I I can tell you why facias rot.

a because they arent painted before the guttering & roof trim are fitted. (don't try to tell me the misting of pink primer that comes on facia is worth anything)
There are little rot groves in my facia where the roof trim contacts the facia.

b because the ends and joints are never primed. ( mentioned before)

c because nails holes are not sealed up or painted over

d because inferior bits of facia are fitted rather than rejected.

In essence because the job wasnt done right in the first place.

cheers