The only test flight so far ended in disaster with the space craft breaking apart going up.
Actually, SpaceShip2 underwent 54 test flights, most of them unpowered. The accident occurred on the 4th powered test flight (the first with the new solid propellent mix).
Let's agree that SS2 has never been in space and any tests (sic) so far has been at low speeds at low altitudes ... and you wonder what they are testing there? That SS2 can glide in air?
Yes. Seeing as commercial aircraft must undergo rigorous flight testing before being approved to carry passengers, I'd say it's quite prudent to test SS2's flight characteristics in unpowered, as well as powered modes.
The engine has only solid fuel for 70 seconds and can thus only provide power for that short time. If it can be stopped and restarted is not clear ... and you wonder why it should have that capacity. To avoid colliding with other space ships?
I don't recall anyone claiming that the rocket motor could be restarted. It can, however, be stopped early for safety reasons if necessary.
The SS2 cannot be steered or brake in space!
SS2 basically follows a parabolic ballistic trajectory. Steering and/or braking while briefly in space isn't really necessary.
The brake and steering only works at say <20 000 m altitude but there is no guarantee that SS2 is stable then. The speed may then be >1000 m/s and SS2 has never been tested at that speed.
That's why SS2 was conducting powered flight tests. Please try to keep up, will you?
It seems the whole thing is a Branson toy to impress boys and girls believing in human space travel.
Why shouldn't a quick ballistic trip to 100km high be possible? What technical reasons can you think of that should make such a venture impossible?