Chris Bennetts wrote:
But that isn't a constraint with a two-man crew, so it does appear
unnecessary.

If the plans were developped at a time when shuttle was expected back at
station in March with crew augment back to 3 in march, then it was wise to
move the soyuz now while they are less busy.

One also has to take a long term look. There may be clearance issues further
on down the line with regards to shuttle and soyuz schedules where it would be
preferable for the zarya/Pirs cycle to have rotated to have a soyus at Pirs
during a certain Shuttle flight.