Basic idea of multirole fighters sounds almost too good to be true: aircraft that can carry out both air to air combat and bomb enemy positions; in some cases, far more. Usual argument for multirole fighters is that it allows for more effective force, since certain number of aircraft will not be split among many different, narrowly specialized types, allowing for numerically superior force, as well as easier logistics.
But how it works out in practice? Answer is: not so great. First and foremost, pilot is most important part of aircraft. Good pilot in bad aircraft will always beat bad pilot in good aircraft; and only way for pilot to become – and stay – proficient is through constant training. Pilot that has to train for air-to-ground cannot spend as much time training for air-to-air, impacting his skill, and multirole aircraft are usually more complex, limiting time pilot can spend training in the air. Technology cannot compensate for lack of pilot skill, as capable pilots will always be able to counter opponent’s technological advantage by forcing him to fight at their terms. This was shown in exercises too, when experienced pilots flying F-15s and F-16s were able to defeat opponents flying stealthy YF-22, despite the latter’s advantage of stealth and unrealistically high Pk assumptions for BVR missiles, as well as agressor’s force own lack of dedicated counter-stealth measures. As soon as it happened, though, USAF put heavy limits on agressors which all but destroyed their capability to counter YF-22, and later F-22, effectively. In World War II, top 5 German aces shot down 1453 Allied fighters; however, it did not help as Allies were able to train far larger number of good pilots to replace losses.
Second, designing true multirole aircraft requires many compromises. Air superiority aircraft needs to be as small and as agile as possible – this results in small size, low wing loading and reduced static stability. But both small size and reduced static stability limit its ability to lug around heavy air-to-ground stores, while low wing loading creates problems during low-altitude penetration missions. Fast jets cannot do Close Air Support, as they cannot fly low and slow enough to identify targets – which was shown well in Iraq when 16 Kurds were killed and 45 injured in friendly fire incident in 2003. Further, while A-10s almost never missed, faster jets often had to make several attack passes to hit target – even when target was identified by ground troops; they are also very vulnerable to even small arms fire. Slow aircraft, for their part, suffer energy penalty when engaged by opposing fighters, which means that, if engaged, they cannot achieve advantageous position and must rely on heavy ECM to survive until air cover can arrive.
There are also many operational limits. Multirole aircraft are often heavier than single-role ones, which increases cost and limits their ability to take off from short dirt or metal-matted strips, which is a crucial ability for any modern air war (only Western fighters than I am fairly certain that possess this ability are small, close air support A-10 and multirole, but also relatively small, JAS-39 – latter doing it by virtue of clever design and close-coupled canards. But even Gripen had to pay for its multirole capabilities, with reduction of IR signature as seen from the ground being paid for by the loss of rearward visibility. Further, its main focus is still on air-to-air performance, with air-to-ground performance taking a distant second place). It allows aircraft to be based close to the troops they support, thus providing quick response, increasing number of sorties that can be flown by certain number of aircraft, as well as allowing aircraft to disperse quickly in case of airfield coming in danger. While this did not come in play in wars fought by US in Iraq and Balkans, only reason for that was huge US advantage in both quality and quantity of pilots, aircraft and ground crews.
Ability to take off from easily-repairable improvised air fields should never be underestimated: in 1971 Indo-Pakistani war, a squadron of PAF Super Sabres did not fly a single sortie as their runway had been cratered, despite aircraft remaining undamaged in underground shelters. There is also no need for STOVL capability: fighter can easily use JATO to take off from no runway if required, and land on grass field after expending most of its fuel and ordnance. This stems from the fact that aircraft spend most of their time on ground – and more complex aircraft is, more time it spends on the ground. Aircraft could also be hidden hundreds or thousands of meters away from runway, and carried to runway with helicopters.
While well-designed multirole aircraft can offer some cost savings, and can thus be logical step for smaller countries that cannot pursue several programmes at once, care must be taken that it does not go too far and passes point of negative returns, as it did in F-111 Aardvark and F-35 Joint Strike Fighter, which both had to adopt incorrect “maneuverability is irrelevant” mantra. In fact, all good multirole aircraft were always air-to-air first, anything else second, if they weren’t simply adapted from “no pound for air to ground” designs (examples of first are Saab Gripen and Dassault Rafale, of second F-16 and Eurofighter Typhoon).