In the Netherlands, Bitcoin is classified no more than as "something with value". As a consequence, it's subject to capital and assets tax, which is based around the assumption of 4% capital gains per year, which is taxed at 30%. So you pay a flat 1.2% of any capital and assets you have over the threshold value (~20K for single people, ~40K for married couples / registered partnerships), regardless of whether you keep it as fiat in a 0% interest account or multiply your capital tenfold by holding bitcoins.
The tax amount due for 2013 is based on ones capital on the first of january of 2013, so the main Bitcoin-taxation won't come for another year, when the price on 1-1-2014 will determine how much you pay for 2014.
There may also be reasons to "mark to market". If one were on the board of an entity that received bitcoin donations and would like to stimulate the discussion among the board of what to do with it.
