EV was using the term "monitor" rather loosely when the S40 came out. They are more of a PA fill or foreground music speaker. It all depends upon what you are doing in the studio. If you are mixing or EQing recordings, and need those recordings to sound good elsewhere, those speakers are not accurate enough. If you are just listening to stuff, then they are decent, small speakers.
The other thought is don't only use one speaker as you're mixing. Lots of studios have Avantone cubes, Yamaha HS8s and then a large far field speaker. Sometimes they're nice Urei's but they can just be any non-studio speaker so you can mix for all types of playback.
Kind of "ditto" to what others have said - it's just another speaker, not aimed at control room monitoring. That doesn't mean it's bad or not useful, simply that it's not a "reference-style" loudspeaker.
You'd want a reference monitor if you're making EQ decisions while recording - you're altering the source (potentially forever if you work with original raw tracks). If you're evaluating mixes use the reference monitors; listening on a wide variety of speakers is helpful to hear how a mix plays "on average". If you're mostly doing casual listening then whatever sounds good to you is okay, too.