Some people play $100 slot machines. A lot of people do not. The risk is very high, and the rewards can be very high, too, but they are not rich enough. The expectation is negative. Thus, except for the entertainment value of stuffing $100 tokens in a machine, it is irrational to play them.
Most seasoned horse players avoid situations like the high-stakes slot machines. They consider them unproductive (negative edge), too remote (low, low chance of winning anything at all) and uninteresting (no skill involved). Serious horse players try to fashion a balance of wagers that are the opposite of the $100 slot.
When there is a good bet "with value," such as an overlay with a palpable chance of winning, the seasoned wagerer may put a fair amount down (let's say up to three times his or her "minimum bet"). This would be a "prime" or "main" bet, and would represent one of say, half a dozen such bets in the course of a day in the race book.
If a comparison of the pari-mutuel odds to the player's own handicapping discloses an edge in a given race, the seasoned wagerer may put down the minimum bet, even though it's a long shot, just to have "action" in the race. "Action" bets might be somewhat high in number, but relatively low in total value. The greater the edge (the difference between the track odds and the real odds, per your handicapping), the more willing you might be to boost the wager a bit. Still, in all, even high-edge bets that are long shots can deplete the bankroll. So they are handled with care.
For fun, it may be worth taking one or two really long shots for a big prize. Sometimes this is referred to as going for the jackpot, or "jackpot bets." These wagers, being the more outlandish of the exotic bets, are sometimes called the "thinking man's lottery." This is because the odds, even though they may have an extremely favorable edge (handicapping when compared to the cost of the ticket), are nonetheless very remote. Seasoned wagerers might go in for one or two high payoff, exotic bets in the course of a day.
Naturally the handicapping is more difficult in combination bets, of course, especially in multi-race bets. To bet on races without proper handicapping is, of course, putting you back into category of the ignorant and visceral fan who just came out to throw away some money.
In all three cases - the prime bets, the action bets and the jackpot bets -- by definition, your edge should be positive. If you had a zero or negative expectation, there would be no reason to bet at all. A positive edge implies that if these wagers were repeated over and over a "large number of times," in the long run, each wager would wind up adding money to the bankroll.
The main difference among them is how long one has to play and how big a bank-roll one has to have in order to be able to hang in there for a winning ticket. The "main" or "prime" bets should be very easy on the bankroll, as they should generate some money much more frequently than the "action" bets or the "jackpot" bets. That is the essential concept of "balance" in the bets made on a given outing to the track or the race book.