Airpower is only a necessity when it comes to INT for defence. However, that does not mean that other planes are useless.
CAS are great and cheap mobile-artillery units that can be deployed at any battle were you need them. They speed up combat significantly and even 2 squads can make a huge difference. Also somewhat useful for naval attacks (to a limited extend, but every piece of combat power helps).
TAC are great allrounders with the ability to both act as combat support and disruption units. Logistical strikes on the enemy will drain his supplies and worsen their overall supply situation, esp. good against island garrisons to assure a -50% combat penalty for your enemy. They can also used to perform strategic bombings, like attacking undefended ports to disrupt repair & resupply overseas or bring down AA and radar.
STR are great for strategic attack (duh) and will bring down IC, ressources and supplynetworks in no time. Yet quite costly unless you get a lot of them, many nations should do fine with using TAC as "cheap STR". Also good for long-range convoy raids out of enemy interceptor range.
NAV are still a bane for ships, although not overkill anymore. Good to have them becaue you can deploy them in any ongoing battle near the coast, increasing you combat odds dramatically. While ships need days or weeks to rush to your attacked fleet, these planes will be there within a few hours.
MRF while rather weak during the early game, they become quite useful from 1942 onwards. Better range, limited ground attack potential, but also a bit more air defence than INT.
CAG well, carrier planes, a bit like MRF, but a bit more focused on bombing. Lack decent ORG/MOR techs atm, but still the only planes that will start bombing the enemy fleet within 1h of engagement start.
TRA are great for both deploying paratroopers and ease pressure for your supply network. Build a few airports in Africa and look how you can bypass severeal low-infra provinces. It's like many little Berlins during the cold war...
Air power IS useful. Just don't expect it to be overpowered like in HoI2. It is now a tool of combined arms instead of a mobile slaughter unit.