Since going to a simple 3 index fund portfolio, I do use a little bit of "play money", but only in my IRA or 401k, as there is no tax assessed on selling stocks or reaping dividends. Of course, if I lose money, I don't get that tax benefit either.
And yes, if you do this multiple times over the course of the year in a taxable account, your taxes will be more complicated. Stocks held less than one year are taxed at your regular income rate (10-39%). The dividends are taxed under a different system (0-20%), so you have to report them separately. Some stocks have non-qualified dividends (REITs) that are taxed as income as well.