It is definitely possible to do long rides on liquids alone. There are a number of liquid nutrition options out there ... Sustained Energy, Perpetuem, and my personal favourite Ensure. I rode the Rocky Mountain 1200 mostly on Ensure, and RAAM riders seem to like Ensure as well. For me, it tastes good (there are several flavours and Butter Pecan is particularly nice), goes down well, sits well, and provides me with energy. You can get the powder to mix up on your own, and it comes in convenient little bottles now too.

These choices have more than just malto, dextro, and electrolytes. They have protein, fats, carbs and a whole list of vitamins and minerals.

My preference among the Ensure products is Ensure Plus: http://ensure.com/products/ensure-plus (you can see on that site the protein and fat content they have).

If you're a fairly fast, fit rider, and can do 60 miles in 3-3.5 hours, you could probably eat a decent breakfast, have one of those 350 calorie bottles of Ensure halfway along, and drink HEED and water the rest of the time.

I can't see any drawbacks to doing a 60-100 mile ride on something like Ensure (or SE, or Perpetuem, or chocolate milk, etc.). The only thing is ... if you plan to travel to other places, especially other countries, to do long rides you may find that your favourite liquid nutrition is not available or hard to get there. Therefore, you may want to train yourself to eat solid, real foods on long rides ... things which you may be able to find in shops along the way.