Like most of its competitors, brokerage giant Charles Schwab welcomes so-called active traders -- and those who might want to become active traders -- with open arms and a list of freebies. Want to take a "webinar" on options trading? There's no charge. Looking for the kinds of stock-picking and forecasting tools that used to be found only on the desks of professional stock sharks? They're yours, for free. Itching to stream quotes and research onto your iWhatever? It's on the house.

But all this generosity isn't motivated by altruism. At Schwab, active traders have an average account balance of $750,000 -- six times as big as the run-of-the-mill customer's. And as the industry has learned, these active types are their most lucrative clients, spending far more on everything from banking and mortgages to, of course, commissions on all those trades. In an industry that has survived lean times since the tech boom, these are the fat cattle. "It's a group whose needs you want to make sure you meet," says Kelli Keough, vice president of Schwab's active-trader group.

Thanks to the financial crisis, the brokers wooing this group have had a more-than-usually receptive audience. After sharp losses, many sophisticated investors ditched full-service brokers in favor of discounters that would let them trade on their own, analysts say; at the same time, many investors lost faith in "buy and hold" investing and considered trading more frequently to catch the market's ups and downs. According to financial research firm Celent, the number of investors who qualify as active -- generally, those who trade at least 36 times a year -- will reach 16 million this year, up almost 50 percent since 2007. Richard Repetto, principal at investment bank Sandler O'Neill & Partners, estimates that this small minority now accounts for about two-thirds of the industry's commissions and fees; in 2011, they generated more than $2 billion in revenue at the four largest publicly traded brokers alone. With so much money at stake, it's no wonder the do-it-yourself crowd has become the subject of a trading-technology arms race. Schwab and TD Ameritrade, for example, have paid a combined $1.6 billion over the past two years on active-trader-related acquisitions.

Still, as the brokerages acknowledge, many of these tools are aimed at encouraging investors to trade more -- and that, critics say, can lead to unforeseen pitfalls and extra costs. Brokerages have been aggressive about promoting trading in options and currencies, for example, pitching them as investments that hedge against the volatility of the stock market. But both are relatively complicated transactions that can lead to expensive trial-and-error periods for newbies -- only about 30 to 35 percent of retail currency accounts turn a profit each year. And commissions on such trades can be higher than those on plain-vanilla stocks. Chris Allen, capital markets analyst at investment firm Evercore Partners, says that investors who use options or currencies are also more likely to borrow money "on margin" to make trades -- at high rates that are highly profitable for brokers.

What's more, keeping the perks of active-trader status can prod investors to be active even when they'd rather stay put. One common arrangement imposes fees on customers who don't make a certain number of trades per month. James Anderson, an unemployed engineer turned trader, says he has often had to buy extra shares to avoid a $99 monthly fee for his trading platform. But trading more often, he notes, has generated tax expenses that might outweigh the fees he dodged. "It may end up a wash," he says.

The brokers say they're seeking a balance between revenue and customer service, using training to steer clients out of trouble. Brokers want customers to make educated choices, says Nicole Sherrod, managing director of TD Ameritrade's trader group; otherwise, she adds, "they can see substantial losses." But no matter how many safeguards brokers put in place, some critics say, one investing rule holds true: More trading isn't always a good thing. A study by Brad Barber of the UC Davis Graduate School of Management found that investors paid a "performance penalty" for active trading -- during one 1990s bull market, when the market earned annual returns of 18 percent, investors who traded 75 percent or more of their portfolios annually earned only 11 percent. Trading too much, Barber says, can "be hazardous to your wealth."