Free member programs have so many immediate perks it's stupid not to join
Unlike frequent-flyer programs, which really only benefit you if you fly with one particular airline a LOT or use an airline-branded credit card for all your spending, car rental agency member programs -- like Emerald Isle, Hertz Gold, Dollar Express, Thrifty Blue Chip, Avis Preferred, Budget Fastbreak -- are free to join, don't require a credit check, and begin accruing benefits pretty much immediately.
In addition to free upgrades/rentals, all member programs allow you to bypass the counter entirely, regardless of how many people are in line or how infrequently you actually rent. In fact, the goal of most rental companies is to get members out of the building (if they have to enter at all) in under a minute. A minute! Plus, members typically score pre-assigned newer, nicer vehicles.