As K. Ueshiba once denied that Daito ryu had much to do with M. Ueshiba's aikido, we must split the world of aikido into two very separate realms.
As Drew and Oisin both note, the modern world of aikido which traces its lineage back to K. Ueshiba and K. Tohei, that world of aikido is a far different entity than the aikido of M. Ueshiba.
In this regard, talking about aikido with both in mind just brings more heated discussions. The modern aikido world spread far and wide and its ideals changed into these notions of spirituality, conflict resolutions, peace, harmony, etc. (NOT saying these are bad things.)
M Ueshiba's aikido was very different and revolved around each individual person becoming something very different martially, such that one lived a bit freer in the world. As M. Ueshiba found when he first met Takeda, all his physical strength failed him utterly and completely. Takeda, martially different, had no trouble handling most people as they relied upon physical strength, timing, body placement, etc. M. Ueshiba learned Takeda's aiki skills and then, because of those skills, it opened a different spiritual door for Ueshiba. A door that allowed Ueshiba to merge his strong sense of spirituality with his strong martial aiki skills.
Modern aikido isn't the founder's aikido. That's neither good, bad, right or wrong. Just different. Modern aikido relies upon spirituality more than martial ability and there will always be heated discussions on its martial effectiveness.
The founder's aikido is rare but making some progress to be reborn. It's martial capabilities are strong, but its spiritual capabilities rely upon each individual person. And each person creates their own level of spirituality. So, more martial than spiritual, in comparison to modern aikido.
The purpose of aikido? Which aikido? Modern? IMO, that's built upon more spiritual than martial. It's worldwide and has devoted, loyal followers. It can be very worthwhile for all the time and training invested. The founder's? That's a tough, rare, and hard road to follow. It isn't for everyone. The martial will outweigh the spiritual for some time and then the spiritual is all up to you. Tough thing to shoulder when handed a boatload of martial power.
But the purpose?
If you're wanting the camaraderie, the focus of being part of something, the group spiritual whole, the feeling of belonging, the training, the spiritual, the harmony, the conflict resolution, etc, then stick with modern aikido. But just don't ever expect that you'll be as good as the founder martially. (For that matter, it won't get you the founder's spirituality either, but K. Ueshiba changed the spirituality for modern aikido.)
If you're wanting the martial abilities of M. Ueshiba, look for Daito ryu aiki. Look in the non-aikido forum. It is readily apparent to those who have experienced this that the abilities of M. Ueshiba are most definitely within one's grasp. Surpassing Tohei, Tomiki, Shioda, etc is not a dream but a very defined reality. But don't expect this to ever give you the founder's spirituality. That's a different type of training, up to each individual to undertake. Without the spirituality, you won't be doing the M. Ueshiba's aikido, but rather another "way of aiki".