The most convincing problem will be, you may end up using stale data or old data or 'others' data, for example if you declared the form in session scope and initialize it with some data, and if within the session expire time out, some other action access the same form, the action gets the same old data as that got initialized by first Action..
Struts don't instantiated the new Form, if that form object is already present in session.
And other problem, like memory consumption.
HTH