I'm sorry, am I the only one shocked when reading "300 MB of RAM" for a software which just plays music? mplayer uses a 6th of that to play videos.
A big part of the client is for browsing through music/album/playlists/artists. Of course you could search for music outside of Spotify but it's really convenient to have it integrated in the client. There's also a social media part (definitely bloat, I have disabled it) and more hidden features like being able to control the client from other Spotify clients (e.g. on my phone) which I use all the time.
300 MB is not great but it's low enough that I don't really care.