As i see it, every team has to have 3 frontline pace bowlers as a minimum, to cover the powerplay and death overs. Beyond that, two spinners are a luxury that can only be afforded if one of your pace bowlers is a proper allrounder or if one of the spinners is an allrounder/part-timer. Even then, the lack of flexibility that this would involve would make it a big risk unless you had one or two other part-timers to fill in as necessary.