That's why those bright heads split the packages between "normal" and "development". It may be the right thing to do for most people. I myself tend to be very very conservative with "modern" technology
The .a is the static counterpart of the .so, so you need the .a if you want to statically link a library (as opposed to using the .so, for which you need the .la). The funny thing is that most programs are developed using the traditional way, i.e. a Makefile that builds the program. make install usually installs everything (yes, including .a and .la, etc.)
The .deb packagers (maintainers I think they're called) take that and generally make a mess of it by adding dependencies, using more-or-less common sense (unfortunately not always) and decide to strip one thing or the other or (in the case of Maemo) to "optify" the package after-the-fact (instead of compiling with the appropriate --prefix). It's actually pretty incredible that things like Maemo, Debian and Ubuntu actually work OK-ish