Well, I can certainly find plenty of agreement there. I have said all along that viability is really the only point where the sides CAN compromise, though there are those on both sides who will choose not to do so.
Because it is arbitrary, many on the pro-life side simply don't want to deal with it, and continue to insist that destroying a microscopic clump of cells is the equivalent of baby-killing.
But it HAS to be arbitrary. To delve into the other argument - accepting that a zygote is a human being with full rights, and then exploring the extent of those rights, I would STILL contend with moral authority that no human with full rights has any right whatsoever to rent another human's body for 9 months, with all that it entails.
I often wonder if people who are so vehemently pro-life have ever even known someone who is pregnant, and what it means to be pregnant physically, emotionally, mentally, etc. It is not something to be taken lightly, and despite the characterizations of women who have abortions as doing so out of "convenience" or as birth control, the situations are very often far more serious than that, and the circumstances such that abortion is truly the best option, for anyone involved.
Viability is a good compromise. It's arbitrary, and there may be different standards for where it occurs, but it's the only sensible option.