How often are we going to be at a bar (where a good many fights get started these days) and bump into a drunk who happens to be 10 years into wing chun? Even in these situations, you aren't going to see the trained stylist fighting under controlled circumstances. What situation can you envision yourself having to face off with someone who might happen to be well trained in another art where it's more than a quick push and a few punches thrown? How many bar fights are going to have a trained boxer circling you throwing lead jabs, measuring you so he can knock your block off? I admit, there are rare situations where it might happen, but rare is the key word here, very rare. We don't live in an age of dojo vs dojo battles anymore.
There is always discussion about whether Aikido is a martial art. When I had the good forune to do some training with Ellis Amdur Sensei in some classical systems he defined "martial art" as training to fight another professional. That certainly was the model for the samurai. They were professional warriors who had every expectation that they would fight other samurai, warriors who had trained since childhood in the arts.
I was taught that Aikido was definitely a martial art though not limited to that alone. So when I train I do so to develop effective technique against the most skilled attackers I can practice against. Most systems of self defense are basically designed to be used in civilian situations in which the attacker may be dangerous but is not expected to be formally trained. These systems are inevitably simplified systems based on more sophisticated martial arts systems. They can be learned more quickly than a true martial art because they have been simplified but they have the disadvantage that against highly skilled attackers they are too simplistic. A classical ryuha was usually a complete system of mutiple weapons, empty hand, strategy etc. A swordsman wouold train , not only against other swordsmen but also against any weapon that a swordsman might encounter. Asking why you would practice Aikido against boxing techniques is like asking why a swordsman would practice against an opponent with a kusarigama or a jo.
In the classical days a student who had completed his training in a given system would often embark on a period of discovery in during which he would engage in challenges with practitioners from other styles. This allowed him to test his understanding against styles different from his own. O-Sensei clearly had the ability to do his technique regardless of the background of the opponent before him. We don't have other ryuha to test ourselves against any more. In fact we don't do challenges or competition. So the closest thing we have to assist us in developing the widest range of effective technique is to keep the Aikido training context but ask the uke to utilize non-standard attacks. Not to do this is to either give up on Aikido as a valid martial art altogether (which many people have done) or to train on the general principles of martial application without really researching their specific application in different types of confrontation. That may the choice of a given Aikido student but they should not be fooling themselves that they are doing Aikido as a martial art.