Never heard of that, but in general, ANYTHING that helps you form connections to other executives is definitely worthwhile.
Maybe a year down the line your phone will ring with a great job offer from the externship place.
At minimum, ideas triggered in a different environment will help your creativity at your regular job (but you've got to be scrupulously careful to not pirate their know-how, their IP, nor to give thenm any know-how from your regular employer. In fact the exo place may ask you to sign an NDA, which you should read very carefully before signing. Strike out any clause requiring you to indemnify them.).