Comparing Seam to Struts is like comparing apples to oranges. Seam is more of a full-blown framework like Spring, but it leverages on JEE standards. Not sure why it's not famous, but it may be because people are happy with Spring and there is no compelling reason to move to Seam. I played around with Seam 1.x long time back. It was better than the bare bone JSF + EJB + JPA, but Spring seemed to be more powerful back to me then. Not sure about Seam 2.x, though.
SCJP 5.0, SCWCD 1.4, SCBCD 1.3, SCDJWS 1.4