First off because unlike the army they can. Also you always trust the guys wearing the same uniform. All Marines are trainded grunts so it is nice to have someone who knows what it is like below above you

Typical Marine. Can't spell (trainded, yo) or realize that the word army should be capitalized because he's referring to a specific army: namely the U.S. Army.

A certain amount practical. A certain amount political. Practically. Years before anyone had really used the term "joint" the Marines were operating as combined units. The goal of the Air Force and Army now are to emulate the Marine Air Ground Task Force concept. However, in the world of ever more integrated joint operations, the necessity of the Marines keeping their own air force may blur. Does anyone really think that the Marines are going to show up at a fight all by themselves? No Navy. No Air Force. No Army coming along behind to clean up (:-0). Navy and Marine tactical air are being combined in a plan called "TACAIR Integration". There will be a Marine fixed wing squadron in each carrier air wing. There will be three Navy F/A-18 squadrons assigned to Marine Air Wings. This does beg the question, why pay for two air forces onboard an carrier? The political part is that the Marines are very good at getting Congress to listen. And the Marines I work with will tell you with a straight face that the reason that the Marines NEED their own air force is because the Navy abandoned them on Guadalcanal in 1942. Huh? Yes, Virginia, we have to pay for not one, not two, but three completely separate fixed wing air forces because the Navy had to save one of its few carriers in the desparate days in the South Pacific 62 years ago. I have endless respect for the Marines, but sometimes, old lessons get learned too well. Maybe its time to move on.

Another question though...does it really matter? The Marines part of the DoN. There's no friction. Both fixed wing aviators take the same training. When operating off carriers, they come under the control of the CAG, in other words the Navy. Are we actually paying for separate air forces? Without the Marine air assets, you'd simply see more Navy air. Also, there are clearly some benefits to having a combined force. For example, when we need a brief intervention against a Third World country, nothing major like an invasion, but a significant military action, the MEUs are THE go to guys. In those scenarios, the fighting would be over by the time the AF or Army arrived. For example, Grenada could have been an all Marine show if necessary. It was "jointed" so all the services could show that, yes, they too served a purpose. There's also the undeniable fact that the Marines have done a good job with their combined assets, and have been quite creative. A lot of the combined arms doctrine you see now is being heavily borrowed from MAGTF experiences. So at the very least, it's like a tactics experiment unit. Organizationally, it's a lot easier for one organization to come up with combined arms doctrine compared to having 2-3 services come together and work something out.

Bascially, my point is why fix something that's not broken? Especially as it's also good for morale? There's also the arguable issue that Marines provide better CAS...A-10 or no. The reason for this is that Marine aviators go through TBS, pick up ground pounder experience, then usually go serve a tour attached to an infantry unit. It's about as close as you can as to putting an infantryman in the pilot's seat(to go along with FACs in infantry units to "talk" to the pilots).

But is it more expensive? Perhaps a lot of money could be saved.

How would it be more expensive? You're talking about the same number of aircraft aboard a carrier, whether they are all Navy or a mix of Navy/USMC. Eliminating one USMC fixed-wing squadron won't allow you to squeeze two extra Navy squadrons onto a carrier. As for training costs, I'm not sure about those, but speaking (admittedly, as a civilian), I'd much prefer having a guy who KNOWS what it's like on the ground flying cover for me, and as a taxpayer I'm willing to pay $10 extra a week from my paycheck (or whatever it works out to) to allow our guys to have the best aircover money can buy.

Exactly. It's the same type of plane, same training, same everything. Maybe you lose a little money sending them through TBS, but that's pretty much it.

are marine air units based at seperate bases or at the same Naval Air Stations as the navy guys?

Generally, speaking no. The marine air bases are all close to Marine Corps ground bases. Like San Diego or Cherry Point+Lejeune.