Not really - as Saab has done this for years. You must have the PREMIUM stereo option for the complete speaker package. Base model stereo car's just had the min. number of speakers. I am in the process of adding door speakers for my 2000 9-3 right now. AMP is also required for this update.
98-06 9-5's were the same way. Subs also only came in 9-5's with the premium stereo.
This logic continued into the Sport Sedan 9-3 era too. Only makes sense they'd do it on a NG9-5