Providing the option to force 'On Behalf Of' contributions would allow useful segmentation of contribution pages into those that are strictly for 'on Behalf of' actions.

For example, if you want to allow an Individual Membership sign-up, and an Business Membership sign-up form, to target the audiences well it may be useful to have different text and diffent options on two different contribution pages. If the 'On Behalf Of' is optional on the Business Member page, it would lead to confusion and the possibility of individuals signing up for business class memberships.

I have implemented this using a hook to set the default value of on behalf of prior to loading, and small change to the core to make the option button hidden.

Would be useful to have a checkbox on the Contribution form configuration pages.

Do the same for Profile / Create CMS User as noted by lobo below.