why isn't it a big hurry?

I think the reason you need an SNB if you are having a mastectomy is that they need to inject dye into your breast to map lymph nodes and find the first one (the sentinel).  That way, they know which one to test. If you have the SNB, in the unlikely event something else shows up on your pathology besides DCIS, they will know (by having done the SNB) whether it is in the nodes.  So it really is a precaution.   At least that's how it was explained to me.  After the mastectomy, you can't map the breast and thus can't identify the sentinel node; iow you can only have a SNB before a mastectomy. 

In terms of the rush, I feel that way too: I am scared of having to have waited so long to get this stuff out of my body.  But I have to say, none of the 6 doctors I've consulted has indicated any sense of emergency.  Mine was found on a mammogram (calcifications), and I figure I could just as easily have had my mammogram done a month later (or earlier).  By "spread," I think you mean become invasive (i.e., no longer be DCIS).  As long as it is DCIS it cannot spread into your body (only harmlessly in your ducts); that is, it can't invade your body or metastasize.  Some DCIS does eventually become invasive; that's basically why we have lumpectomy or mastectomy--to prevent that.  But as long as it's DCIS it can't spread.  I figure doctors these days are very cautious (defensive medicine and all that); if they think what they see is in danger of being invasive, they would probably act more quickly and with greater urgency.

 What I have written above is what I have been trying to tell myself for about 6 weeks now.  I go back and forth between being really scared and feeling pretty safe.  But, like you, I want to get it out.  I want to know the final pathology.  And I want (I am hoping,  hoping, hoping) to put it behind me.  

DSJ did a great job of giving you the facts, exactly as my Dr. explained to me. But you are not being a wimp - this is a scary and confusing diagnosis. On one hand I felt "lucky" to have a non-invasive cancer, on the other hand confused because my choices in treatment all seemed so drastic  compared to the diagnosis. It took me several weeks to get all my information straight and sort of digest what was going on. By the time of my surgery - Mastectomy and flap reconstruction I was OK with my decisions. These decisions are permanent, so it's important to take your time and not be rushed by anyone, family, friends or Dr. Good luck and hope you find peace with your choices.