I'm a 4th year DO student and I had the same questions as to take the USMLE or not. I wish I could go into the 2nd year class and tell them the importance of taking usmle BOTH step 1 and step 2! If you are applying to the allopathic match, having the usmle will allow them to compare apples to apples. For my interviews, I was told too-many-times-to-count that taking the usmle makes me as competitive as my MD counterparts.
I think where it comes in most handy is to places where you did not rotate, and also when it comes time for the programs to rank the applicants.
Also, I think programs really like when you see that you took the usmle. I went through and struggled as to should I take usmle or not---and looking back at it, it was one of the best decisions I made. I applied to 40 allopathic EM programs, I got interviews at 32 of the programs. Besides my SLOR's, I truly believe that number was high because of the USMLE.
As far as taking step 1 or step 2--take both! I did terrible on my step 1, so I was really nervous to take step 2. I studied as hard as I could, and did very well on step 2. Programs like to see that, and I heard from many programs that Step 2 is more important than Step 1.
If anyone has other questions, PM me. I hope that helps and drives home the importance of taking the usmle.