I put rails on mine for a vert grip and a light and when I qualify to throw my bipod on there.  The free floating part is just an added benefit.  When you have a barrel that short every little bit helps.

With the amount of crap people hang on their guns, and the penchant for 'pencil thin' carbine bbls, free float makes sense...

Keeps the crap from affacting your POI by stressing the bbl...

This one's getting a Troy MRF-C when I get around to completing it (CMT upper, Bushmaster 11.5"/5.5" 1/7 HBAR).  The FF benefit probably isn't seen in setups like this, but the added flexibility of the rails and the items that can be added is what I am after.  I'm not looking for long range accuracy with this one, it'll be lucky to see targets past 50yds.  

[scratching chin thoughtfully]I've been comtemplating breaking my EOthingy cherry and mounting one on this upper............[/scratching chin thoughtfully]

The high quality non-FF rails like the KAC sometimes cost more the than the FF rails.   Check the price of the M5 RAS vs the full length FF-RAS. So why not go FF save some money?

Having a FF rail certainly does not detract from the carbine and as NAG mentioned the freefloating does offer certain advantages.

Last but not least the only reason anyone needs, because I want to and because I can.

IMO, the primary advantage of these newer handguard designs is that they usually have a lot more vent holes allowing much better cooling.  Free-floating the barrel is something that newer designs are including and is nice to have though not necessary.

Quoted: tinypic.com/3gd4g This one's getting a Troy MRF-C when I get around to completing it (CMT upper, Bushmaster 11.5"/5.5" 1/7 HBAR).  The FF benefit probably isn't seen in setups like this, but the added flexibility of the rails and the items that can be added is what I am after.  I'm not looking for long range accuracy with this one, it'll be lucky to see targets past 50yds.   [scratching chin thoughtfully]I've been comtemplating breaking my EOthingy cherry and mounting one on this upper............[/scratching chin thoughtfully]

Now I have a question! I'm not saying what you're doing is stupid, but what is the point in having an 11.5" barrel if you're just going to throw a 5" or 6" flash suppressor on it? Wouldn't it just make a lot more sense to have an 14.5 or 16" barrel? Or is it just for looks?

Quoted: Quoted: tinypic.com/3gd4g This one's getting a Troy MRF-C when I get around to completing it (CMT upper, Bushmaster 11.5"/5.5" 1/7 HBAR).  The FF benefit probably isn't seen in setups like this, but the added flexibility of the rails and the items that can be added is what I am after.  I'm not looking for long range accuracy with this one, it'll be lucky to see targets past 50yds.   [scratching chin thoughtfully]I've been comtemplating breaking my EOthingy cherry and mounting one on this upper............[/scratching chin thoughtfully] Now I have a question! I'm not saying what you're doing is stupid, but what is the point in having an 11.5" barrel if you're just going to throw a 5" or 6" flash suppressor on it? Wouldn't it just make a lot more sense to have an 14.5 or 16" barrel? Or is it just for looks?

To answer your question - NEITHER

It's definitely not for looks - I think it looks a little odd, myself.  If I could have an 11.5" SBR, I would - the powers that be here in MI have decided that I cannot have one.  It is the only legal way I can "play" with a Commando length upper.  I am reloading 77gr SMK loads and I wanted to see what they can do through this SBR configuration.  I plan on chrono'ing the loads at the muzzle and at the target - 7, 15, 25 & 50 yds.  Then I'll try different loads and see if I can get the effective range out a little farther.  It'll be a different and challenging experiment for me.

They're much more rigid than simple hanguard replacement rails (like the old Knight's RIS).  Put a vertical grip on an old one and it and it wobbles from side to side, might even make a little rattle noise.   Free floating systems are rock solid and quiet.  

Bingo!

I never use to think that free-floating a 14.5'' M4 profiled barrel was much of a benefit (accuracy wise) either! Until I did a little test.

I was sighting in a new FF upper with an Aimpoint (bench rest and sand bags at 100 yards).

After I got it dialed in I conducted the test. I now rested the Bayonet lug onto the sandbags instead of the FF forearm.

I was only applying a moderate amount of downward pressure onto the rifle (a little less then if I were using a sling) and now my shots were stringing between 2'' and 2 1/2" upward!

I was very surprised to see that amount of deflection taking place from only a moderate amount of downward pressure being directly applied to the M4 barrel!  

The M4 barrel profile DOES benefit from Free-Floating.

Something about have and not need than need and not have