I came to political science (theory) late in my college career but decided to apply to PhD programs anyways. That was a mistake. I was an unformed candidate with little working knowledge of the field or its vocabulary and I was shut out (though, shockingly, initially wait-listed at Michigan). Fortunately I had also applied to my alma mater, University of Chicago, and was referred to MAPSS program, partially-funded. I was initially hesitant (MAPSS doesn't have the best reputation among snarky U of C undergrads) but I am so glad that I looked past some unfair biases and went for it.
That I knew the University well helped make my MAPSS year especially focused and fruitful -- but even students who are initially overwhelmed by the school and forced into making important decisions very, very quickly CAN and DO benefit tremendously.
My experience echoes what's been said here: you get out what you put in. Dabbling in different classes and fields will quickly neutralize the value of the MA (for graduate programs). But sticking to a sub-field, taking advantage of generous faculty, seminars, workshops, etc. will put anyone in a good position to go on the PhD.
I should also plug the advising system MAPSS has devised to ensure its graduates place well in programs. An adviser with MAPSS (also an instructor and recent U of C PhD) works very closely with students to help make sense of, shape, and ultimately narrate their interests into a succinct and compelling research program. They have nearly 100% *funded* placement rate.
Not only has MAPSS given me some excellent options for my continued graduate research, it's also, as a program, a really valuable place to find what you're interested in and pursue it very rigorously for a year. It made me a better PhD candidate and a better scholar.