Paul
WW1 battlefields are being dug for two reasons; the first is to clear munitions that may remain unexploded - there is still one unlocated mine on the western front that contains 10,000 tons of unexploded dynamite, a mine dug by British sappers beneath German lines that failed to detonate. When two similar mines were detonated simultaneously, the explosion was heard in Kent and felt by the Prime Minister sat in his office in Downing Street. Were that mine to detonate today, who knows how many would be killed?
The second reason is to identify bodies and reinter them in proper cemeteries. This is important to some for personal reasons, to others for historical research, and to families for obvious reasons. Just last year a mass grave containing the remains of 250 Australian and British casulaties was discovered at Fromelles in Northern France; the Commonwealth War Graves Commission is nearing completion of building the first new CWGC War Cemetery in 50 years on the site, and reburial on the bodies commences on January 30th with each body being given a burial with full military honours.
http://vimeo.com/8713231
This video shows the work that is nearing completion.
As for the larger debate on how long remains are sacred, for the answer is for ever, in exactly the same way as any grave is sacred. Where the graves pose a potential risk to lives today, then of course they should be moved, as with the example of this German submarine. Moving the remains does not necessarily mean disrespect to the memory of those who died there, in fact it may mean that the bodies can find a more suitable resting place. For me, this is better than turning a war grave into a tourist attraction, as per the USS Arizona in Pearl Harbour, the very concept of which is wrong in my eye.