For using Spotify in a car with FM transmitter of the Nokia N900, you'd still have to use the offline modus of Spotify or Streaming over 3G or Wifi (if you carry a 3G/4G) router in the car, don't you?

Actually I skip this Spotify step and just use the huuuuge memory expansion capabilities to have the Nokia N900 deliver music as a stand alone server with FMtx capabilities. Sorry for all the Spotify goodness but I have hundreds of other music not on Spotify/

-For live streaming , the standard music player for as internet radio client can back up for Spotify and much likely surpass it as a NEWS source & music discovery station.
internet radio (god gift in N900 as the buffer in this one is really good, playing uninterrupted on low quality internet connections)

-Standard or other music player for MP3/FLAC. All can be played over Aux in of your car amplifier or the FM receiver.

-Panucci "resuming audiobook and podcast player" is an excellent application to read your audiobooks (converted to stand alone format by your own "creative method") that has easy way to return to missed parts as well as easy FM tx function.
This and other apps show why its so much worth to port Maemo Leste to all modern devices that will accept it. Sailfish simply lacks this kind of ever lasting useful apps.

-all of the above are of course possible with BT headphone instead of car radio. My Senheiser MM100 allows returning and pausing.

Regarding Spotify in cars, the above scenario's make me not miss Spotify as a music discovery radio station and as a bonus I don't have to care about mobile data usage of even low end 128kb stream of Spotify.