This is only my personal opionion, but if I had tried to solo with my 3D plane I would have needed to repair it every weekend, and then it would have been as heavy as my trainer, even though it only took a month of weekends to solo Our runway is 40 feet wide but it sure looked small the first few times I tried landing on that piece of asphalt!

In anycase, though I only really needed the trainer for a month, I still have it for the following reasons,

-to go float flying
-the times I just want a relaxing afternoon
-introductory flights
-practice weird stuff like side-slipping the wrong way, inverted flat turns etc.
-fun-fly's
-when flying at an unfamiliar field
-extreme wind conditions, harrier landings are fun
-backup plane
-close formation flying plane

In fact I was racing this plane, I know, I know, I shouldn't have been, and the fuse became a lawn dart when the rubber bands went. Since the wing survived, I bought another fuse on ebay. Also, I've done a lot of instruction, and must say that for the majority of people a good strong, relatively stable, and slow flying trainer, is the way to go.

One last thought, the trainer is so much simpler too. Engine exposed and easier to work on. Only 4 servos. Even still it can be a pain to find a suitable buddy box, this would become even more so with the dual aileron/elevator setups in some 3D machines (unless of course one happens to have a 9C).