They say the intended timing of the strike, which affects only that one party conference and not the others, would be "counter-productive".
What this probably means is that they realise that they will be seen as politically biased (which the BBC obviously is, of course) and that could mean that the BBC's current freedom to act with partiality might end up being curtailed as a result. Although the minister (Jeremy Hunt) seems a bit wishy-washy on dealing with the Beeb, he could be encouraged to harden the government's attitude towards the national broadcaster.
Perhaps those journalists are also aware that they won't be able to put their partial slant of reporting the conference at the time, and other (less partial) channels will provide a more accurate and even-handed standard of coverage than BBC people would like. That would explain the term counter-productive!
The BBC's Director General has since echoed the letter's sentiments.
Update: the strike can now been called off (as at 1 October).