As the old saying goes: Perfection is the enemy of good.
I can see why democrats should be opposing the Hagel nomination; he's a republican. But republicans should consider the alternatives before they toss out Hagel because Obama's not going to name anyone more conservative.
The wise choose their battles carefully and in my opinion, there is much more for the republicans to lose than to win by opposing Hagel. I predict he will be rather easily confirmed.
In general, I think that any president should have a relatively free hand to make cabinet appointments. With Hagel, however, there are two problems. First, what sort of signal does his appointment send to Iran? And, second, what sort of signal does appointing an apparent anti-semite send to American Jews?