I technically do that, but I wouldn't say what I do is simple. I use a database, and can just "make" a new portfolio when ever I want. No new accounts, just have to assign the new buys to the new portfolio in the database.
I suspect it would be too much work for most people.
You best bet is probably a spread sheet with separate tabs. That way you can separate your "portfolios" without too much hassle.
Now if on the other hand what your wanting is to track all your portfolios, yea personal capital (PC) is probably best. There are a few others out there, but that is the best of them as far as I am aware.