I don't think it's uncommon at all among Indonesians and Malaysians to say "I speak Bahasa." See for example here and here. I've always had the impression that it's non-Indonesians/Malaysians who want to show they know what "bahasa" means and will insist that it's wrong.
Saying just "bahasa" may also help in Malaysia, where, if I understand it correctly, whether you say Bahasa Malaysia or Bahasa Melayu is a political can of worms all of its own.
See also this link ("And how will the government want to encourage non-Malays in Singapore and other countries to speak in Malay when it is described as Bahasa Malaysia, anyway? No wonder, many of them just say ‘bahasa’. Even on the covers of many Hollywood films, they only say ‘bahasa’ to refer to both ‘Bahasa Malaysia’ and ‘Bahasa Indonesia’.")

