For one thing with mono, you're not splitting the resolution between two channels, which
over all makes mono recordings much denser. Also as mentioned, many early recordings were only recorded in mono or if a record was recorded in both mono and in stereo at the same time, often the mono recording is preferable. For example, Jefferson Airplane's 'Surrealistic Pillow', the stereo version of this record has special effects added to the recording to make it sound psychedelic but the mono does not. I prefer the mono as it sounds more folk-ish.
My digital front end is my higher end source and I'm still building on it so I bought a relatively inexpensive mono cartridge, the Audio Technica-AT 33 mono MC cartridge;
http://www.ebay.com/itm/AT-33MONO-Audio-Technica-MC-Type-Moving-Coil-Monaural-Cartridge-10ohm-F-S-wT...This is a very good sounding mono cartridge for the money and it is a true mono cartridge, so it's possible to play stereo records with it if one chooses.