I watched a webcast today which was critical of photographers who try to do everything. While I agree you are not likely to be excellent at every type of photography I see no harm in specializing in a few types.
For me I shoot mostly music, travel and animals. Those are my main focuses. As has been pointed out to me music doesn't pay and there are challenges getting credentialed to shoot concerts. Recently I asked for photo credentials as a freelance photographer. I was told no, not because they didn't like my work, but because they only give passes to people on assignment. The publications would rather not put people on assignment they would rather take submissions from freelance on a lot of cases and especially for the smaller publications. So this is a loop.
For me I picked these three areas because they are the areas I am most passionate about in my daily life. It fit to combine them but as I have said before I will shoot other things to practice my skill.
I understood what this webcast said when they pointed out you are likely to only be hired for one type of work and not all the areas you shoot. I think this may be true that you are limited to paying work in one or two areas but I still do not agree with them that you should limit yourself to just this. They expanded this into their comments on portfolio reviews. They feel a portfolio should only have one clear subject. Mine is not that. My portfolio focuses on my 3 core areas and only those. I do post other photographs I have taken on my website though. Is that a portfolio? Possibly but not in the traditional sense.
To me it is personal preference. I agree you may excel only at one area but why not try some of the others if you have the time and desire. If you belong to a photography club or participate in photography contests they even encourage people to try other things.
Why did this podcast cause me to think so much? Possibly because I am sitting here packing my gear for the weekend and it is a busy and diverse weekend of shooting. I have a rock concert tonight, a classic car show and a rockabilly music festival the rest of the weekend. Diverse? Yes and I enjoy that. Photography has allowed me to step outside my normal shell and do things and go places I wouldn't normally. This is amazing to me and I wouldn't change anything about it. So I'll pack my bag and go shoot anything I enjoy and I will put any photo into my portfolio if it is warranted.