I agree with you, deekay911. I have this dream that producer Benny Andersson might one day be pursuaded to release an instrumental-only version of the Mamma Mia! soundtrack. That the wonderful Rutger Gunnarson and so many of ABBA's original backing musicians could be assembled 30+ years later to recreate these tunes in a state-of-the art digital studio was to me an outstanding achievement and many of these instrumental tracks sound amazing. S.O.S. for example, which I've long believed is one of the most perfect pop tunes ever written with its innovative mix of major and minor keys, acoustic and electric instruments, and classical and rock influences, is so incredibly rendered on this sound track it truly comes alive.
As a sidebar, the notion that ABBA was a disco act is a peculiarly North American idea, where the band's success was more limited than in the rest of the world and where Dancing Queen is their most-remembered hit. Ironically Dancing Queen doesn't have a disco beat and never made the Billboard dance chart. In fact ABBA only ever placed two songs on the US dance chart. Voulez-Vous reached number 1 there; the classically disco Gimme, Gimme, Gimme never charted and was largely unknown before being added to the stage version of Mamma Mia! and sampled by Madonna.