My take on it.
Are they useless?
Yes they can be useless, if you dont know how to train with them. People seem to think you learn a form and that’s it you can fight. Forms are a training ‘method’. If you train right then you get something out of them,
otherwise its just a dance.
Are they just for show?
No, forms are as lots of people said;
The database of the system,
The seeds of a system,
The handbook of the system,
The genetics of the system,
Add your own metaphor here.
Can you become proficent by only practicing form?
No, you cannot. Fighting is fighting, its dynamic and open. You get good by
fighting alot, by testing what you know
in a non-stylised and free-form way. A
good system provides a structure in going from learning the core prinicipals
to appyling them in a free for all.
Can you become proficent without form at all?
Yes, you could just practise drills,
two man sensitivity exercises and sparring and be excellent, but you need a partner. On your own, you’d just end up making your own forms anyway 
Who is form and why is everyone always complaining about him?
No idea.
Are old forms better than new ones? If so why?
Yes, and no. Yes because any old form is the compound knowledge of all the previous teachers. You need a good instructor to help unlock the secrets, but alot of thought has gone into creating the form. In theory new forms would be better than old forms as more knowledge has been added, but in practise they have been made up with only a limited understanding of the previous systems.
What is the purpose of this ‘form’?
Does having more forms make you a better martial artist?
No, quality always over quantity.
Does the length of the form matter?
No, its the quality of the form that matters. A good form is highly concentrated, everything is there, it has a structure designed to facilatate
learning the key prinicpals of the system. It also has methods which allows you to develop the attributes required by the system.
What elements are required to name something a form?
Form, is just that a sequence of movements. What makes a good form is the quality of the thought gone into constructing it, the techniques, the combat prinicpals behind it.
Oh well… written an essay, again.
Andrew