Why all the confusion? Well, in our case it is from different rims, tires, and tubeless tire systems as well as different riding styles and terrain. In the end, I do recommend tubeless tires to anyone who wants higher performance and less flats but doesn't mind a little extra installation trouble and maintenance.
With the right setup, going to tubeless tires will improve your bikes performance. This is especially true for riders who have to run higher pressures to prevent pinch flats.
I recommend using an internal tire sealant such as Stan's No-Tubes for a more robust system and less flats. I still recommend this even if you have tubeless specific rims and tires.
If you use a tubeless kit to convert your standard tube/tire system into a tubeless tire system make sure your tire, rim, and kit are compatible. Check the web site of the tubeless kit manufacturer for compatibility.
You can use non-tubeless tires if you use an internal sealant but don't use super-light tires with thin sidewalls. Thicker sidewalls provide better cornering performance and if you ride in terrain with sharp rocks they provide better protection from sidewall cuts and tears.
You will still need to carry an extra tube and pump. All tubeless tire systems let you put a tube in if you get a flat and you can't get your tire to seal up again.
If you try to lower your tire pressure too much, you will be more likely to damage your rim when you hit rocks and you may feel the tire roll under during hard cornering. When this gets really bad, you can burp air out and end up with a flat, unsealed tire.
Pay attention to and follow the installation instructions carefully. Take the necessary time to get compatible products and to install them correctly.
A properly installed tubeless tire system is capable of handling any condition and riding style. I have raced both cross-country and downhill with tubeless tires.
From the performance standpoint, tubeless tires are hard to beat. Tubeless tires don't pinch flat so they let you run lower tire pressures. Lower tire pressure is the best way to improve a tires contact with the ground and with that comes better bike performance. That said, tire pressure is one of the most influential adjustments you can make to your bikes performance.
Tubeless tire supporters claim that rolling friction is reduced in a tubeless tire. While I can't say I noticed either way, I do think they have some valid arguments to support this.
Using an internal sealant is well worth the little added weight. Tubeless tires still get flats from thorns and other punctures. It is in most cases more difficult to fix a flat in a tubeless tire than a standard tire.
Compatibility is a big issue. Choose the wrong tires or rims and you will end up blowing your tires right off the rim either during installation or on the trail.
While it is tempting to go with the lightest tires you can find it is more important to get a tire that will perform well and won't end up forcing you to put a tube in later. No amount of sealant will plug a good cut or tear in a tire sidewall.
Don't expect to loose a huge amount of weight. Some systems are lighter, some heavier, it all depends on the system and the tires used. The real benefits are better performance with lower tire pressures and fewer flats.