There are a lot more pitfalls for the spinner than for the trundler. Batsmen's eyes light up like slot machines. Meaty bats and chunky forearms conspire to send good balls into the stratosphere. I was once hit over a spotlight. But there is nothing to match the satisfaction of out-thinking a batsman - holding one back for the caught and bowled; bowling a drifter to the batsman looking for turn; really putting a rip on one for the batsman looking to leave: it's much more strategic than pace bowling.
Since the level of physical exertion involved in bowling slow is significantly less than when bowling quick, more subtelty can be introduced into the action. Look at three examples of Robert Croft bowling to a left handed batsman.
To be honest, the spinner is now so rare in English cricket that simply coming in off a short run will have a lot of batsman fidgeting in the crease uncertain whether to stick or twist. because there isn't the pace on the ball, the safe deflection shots aren't available - to get runs the batsman has to put the pace on himself. Bowl full and just outside off stump and the only safe attacking shot is the straight drive - easily defended. To get runs the batsman has to take risks - and that's always a good time to be bowling.
But it's not as if spin bowling is easy - have a look at the three best English left arm spinners of the last 20 years, Phil Edmonds, Phil Tufnell and Monty Panesar:
Not exactly effortless...