The airport director in Worcester, Massachusetts, felt that Allegiant reneged on a commitment to serve the airport for five years given the use of federal grants to assist its startup. However, the airline responded that the market was immediately unprofitable and starting service there was a poor decision; flights were reported to be 80% full. Allegiant's flights average 90% full.
In January 2010, the airline celebrated its one-millionth passenger to fly out of Phoenix-Mesa Gateway Airport. Allegiant's parent company also announced that it had purchased 18 new MD-80 aircraft from Scandinavian Airlines. In February 2010, Allegiant opened its ninth base at Grand Rapids' Gerald R. Ford International Airport in Michigan. The airline based two McDonnell Douglas MD-80 aircraft in Grand Rapids, but ended their airport's status in 2011. The airline continues to fly out of Grand Rapids in a reduced capacity.
A November 2016 analysis by the Tampa Bay Times noted that Allegiant's planes were four times more likely to have in-flight failures than other major US airlines. A 60 Minutes report by CBS News in 2018 continued to investigate the issues first raised by the Tampa Bay Times. Public records cited in the investigation found that the airline had more than 100 serious mechanical incidents between January 1, 2016 and October 31, 2017, including "mid-air engine failures, smoke and fumes in the cabin, rapid descents, flight control malfunctions, hydraulic leaks and aborted takeoffs." Other public records further revealed that Allegiant aircraft, on average, were nearly three and a half times more likely to have mid-air breakdowns than American, United, Delta, JetBlue and Spirit.
Allegiant aims primarily to serve leisure travelers, particularly those in colder northern climates, going to warm-weather tourist destinations such as Punta Gorda, Tampa Bay, Las Vegas, Orlando, Los Angeles and Phoenix. It also serves smaller destinations that see few direct flights by major carriers. Many of the airline's markets, such as Peoria, Illinois, are served only by commuter service requiring a connection at an airline hub. In October 2009, Allegiant had competition on just five of its 136 routes.
The TWA Hotel is the TWA Flight Center structure currently being rebuilt as 505-room hotel, preserving the Eero Saarinen headhouse while replacing the structures on either side of the headhouse. Situated in front of JetBlue's JFK terminal, JetBlue has stated that it estimates the ownership of the hotel would be between 5–10% of the final total investment. The hotel will be an effective replacement for the Ramada Plaza JFK Hotel on the north end of the airport grounds in Building 144, which closed in 2009.
In 2013, Allegiant acquired nine Airbus A320-200 aircraft from Spanish flag carrier Iberia. Seven of the A320s were delivered in 2013 and were used for growth into new markets, including destinations in Mexico and the Rocky Mountains, as well as airfields such as Charlottesville and Shenandoah Valley in Virginia and Trenton, New Jersey. On February 24, 2015, Allegiant announced the purchase of 2 additional A320s from Philippine Airlines which entered into service in 2015. Later in 2015, the airline announced a firm order for an Airbus A320 direct from Airbus, the first time it has purchased new aircraft from the supplier.