Importance of EM to microwave design?
If you want to do microwave circuit design (amplifiers, for instance), you should be comfortable with E&amp;M theory.  While a lot of the new tools out there (ADS, field solvers, etc) take a lot of the grunt work out of microwave circuit design, a good grounding (pardon the pun) in E&amp;M theory helps understanding the results the tools give you.

If you end up dealing with interconnects, antenna elements such as notch or patch radiators, and antenna arrays, then it's all about E&amp;M.

Unfortunately, I'm not familiar with Pozar's book so can't venture an opinion there.

To be honest, I just got by in my undergrad E&amp;M course.  Now I wish I had paid more attention.  E&amp;M theory is all based on basic principals of physics and never goes out of style or is overtaken by some new technology.  The tools may change, but the underlying principles remain the same.