I think there are three really good reasons to try and replicate an amp sound.
* The sheer challenge of it from an engineering standpoint, and what you'll learn from attempting to do so.
I'd go along with that if you were working on replicating an amp sound from parts - that is from plugins and hardware if you got it that go into making an amplifier sound like it does. But when you load a Famous Fenders plug-in and click on 1972 Twin, they've done all the work, leaving the controls for you to twiddle to get the sound, if they've done a good job, of some particular 1972 Twin that you like. But you're not going to get the sound of a Crate, an Orange, or even an orange crate with a Dynaco 60 tube amplifier and a greenback speaker.
I appreciate and applaud the tips Craig has developed for making a replicated amp sound more like a real amp in a real room. I think that's an important direction to pursue. But it goes beyond "replicating an amplifier," just as an engineer in the studio might do with the 1972 Twin that you brought in for your session.
* You have a signature sound in the studio based on an amp, play live, but don't want to carry an amp around with you.
I suppose there are some for whom that's the case, but most of the artists who have a signature sound based on an amp probably are successful or resourceful enough to have someone else to carry the amp. But this is a good job for a Kemper. It's what it does, or at least what it's supposed to do, and from what I read, does a pretty good job of it. It's not the same as a brand name plug-in, it's your sound with your controls and settings, speaker (?) and gain structure. I understand that it takes some fiddling to get out what you put in, but once you have it, it's your sound.
And, trying to beat Kuru to the punch here - "signature sound" is often more in the hands of the player than the equipment. A good replication would have similar dynamic response to a player's technique, but there's a lot involved in it, so it's not compression, expansion, or such. Is this one of the things that these gadgets use as an input?
* Manufacturers who need to accommodate customers who think "If my amp sounds like the one Jimi Hendrix used, then I will be Jimi Hendrix."
Best reason so far. Take their money and develop an amp that sounds like somebody else.
None of those constraints apply to me, I just want a cool guitar sound
That's probably going to take you more into creative use of effects rather than replicating a specific amp unless that amp has just the effect, tone controls, and control over distortion that's cool for the song you're using it on.