in my experience, most regular PA speakers are voiced for FOH. they have a lot of bass and treble and are designed to throw the sound far. many can do double-duty as a stage monitor as well, but those characteristics are still there. some have buttons or dials to go to "monitor mode" to help compensate for some of these EQ issues.
many FRFR speakers are more ideal for a monitor setup, as they don't push these frequencies from the start. generally, you have to add what you want to these speakers.
majority of music gear is designed to sound good "out of the box" but also at the music store. if you take a stock FRFR speaker and listen to an iPod through it, it won't sound good. then use a QSC K10 and WOW it's sounds so much better, more bass, more treble... i'll take 2! but that can bite you at the gig when you suddenly turn up to 100 dB and you can't control anything.
i own a Flat Response speaker (RCF NX12-SMA) and i love it. it's a bit big for my weekly gigs, and i enjoy running stereo but only have one. so i bought some QSC CP8 speakers. i've previously used QSC K10, Yamaha DXR, etc. - a range of "normal" PA speakers. they are all used as monitors for both guitar and vocals in a duo setup.
the RCF speaker is way easier to use at the gig - i can leave it flat and not worry about feedback, unless the room is completely horrible. then i add some bass to the output EQ if i want it, add treble, etc. i rarely have to notch out any mids.
the QSC speakers sound great, but i have to cut a bunch of frequencies at different areas to compensate for a lot of bass and mid bumps (which help vocals cut through). once it's dialed in the way i like, it sounds great. but it takes more work. this work though i feel is par for the course for any sound system. i choose to leave the speaker's EQ dial set to normal/mains, rather than constantly changing them depending on the situation. then my setup is completely repeatable. (i use a Digital Mixer which can save all that EQ per gig.)
so i'd say that true FRFR speakers are best for monitoring because they don't add their own preconceived notion of EQ to the situation. that said, regular speakers are easier to get and replace, and you just need to be able to EQ them as needed.
also know that "FRFR" is being thrown around these days to mean various things. many speakers are "Full-Range" but not actually "Flat-Response."