Aluminum Welding:
Yes, it generally takes a spool gun to feed the wire reliably, but it can be done with a short enough standard MIG setup with the harder Aluminum wires. Besides the equipment, the gas (Argon) can be a bit more expensive as well.
WARNING - Why Not to Weld Aluminum Even if You Can!
If you are randomly welding things onto structural Aluminum frame members of your trailer, you will destroy the heat-treated strength properties of the alloy in that area, turning it from your nice strong 6061-T6 or 6063-T6 into O temper. If it turns out to be a critical section of the structural part, it could later fail. The only way to make the welds in Aluminum as strong as the original heat-treated alloy is to re-heat-treat the assemby after its welded. Generally impractical for something as large as a trailer unless you are an aircraft manufacturer and have some really big ovens.
Even though holes also weaken and concentrate stress, I think you are better off carefully drilling holes (with a sharp bit and lots of coolant/lubricant so as not to heat the part) and bolting or riviting the extra stuff on.
Stress Cracks:
Are always due to poor design or overloading beyond the intended design. A proper design would limit the repetitive stress and vibration in a given member to stay well below the repetitive stress threshold of the material in question. This is a well-understood property of the aluminum in aircraft design, for example. Some trailer manufacturers may not pay any attention to this. They go by a rule of thumb or use the ultimate strength instead of the repetitive strength. They test it with the rated weight once, and call it good. Their products may fail due to stress cracking. Especially vulnerable are welded connections between frame members. A: Stress tends to concentrate there due to the geometries involved, and B: The metal is the weakest there due to the temper being lost due to the welding.
Why to use Aluminum for a trailer:
1. Lower Trailer Weight = Higher Payload for a given towing vehicle max total towed weight. My '91 Dodge Dakota, for example is only rated to tow 5000 lbs. If I were to try and use a 3800 lb tilt-bed steel equipment trailer, I would be limited to a 1200 lb tractor to haul around. If I instead use a 1000-lb, non-tilting, Aluminum trailer, I can put 4000 lbs of tractor and attachments on it.
2. Safety - the lighter the trailer/load combination, the easier it is to control the vehicle and trailer.
3. Fuel economy - but as one poster pointed out, this only turns out to be a win if you put on enough miles hauling the trailer to make up for the higher cost of the Aluminum.
4. Cosmetics - the aluminum will age more gracefully. Yes, the steel may remain structurally sound, but is is going to look bad after 20 years unless you re-paint it. The Aluminum will dull with age, but will generally not be painted, so there will be no paint blistering and peeling off.
5. Ease of storage / moving by hand. If you ever have to move a trailer by hand to put it away in your stoprage area, you will appreciate the aluminum.
Why not to use Aluminum for a trailer:
1. Money is always an object.
2. Familiarity.
Design and fabrication requires different knowledge, skills, and tools than the more-common steel fabrication. Those not familiar with building with aluminum are likely to produce a sub-standard product if they don't take into account its differing properties and design and fabricate accordingly.
3. Sometimes more weight is an advantage. For example, if a dump-trailer was made super-light, it might tip over easier if raised on a slight side-slope.
