Rudyard Kipling's poem "If" reflects upon the need for a balance in one's life, attachment with detachment, righteousness without self-righteousness, etc. So, too, does Thoreau suggest a balance in one's life when he writes in "Walden,"
I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach.... I left the woods for as good a reason as I went there. Perhaps it seemed to me that I had several more lives to live, and could not spare any more time for that one. It is remarkable how easily and insensibly we fall into a particular route, and make a beaten track for ouselves.
So, to spend two solitary years in a natural setting seems a bit beyond moderation. Yet, some time alone away from the "world [that] is too much with us" as Wordsworth wrote, would probably be good for most people's souls. Certainly, communicating with Nature and with one's inner self is always healthy. Retreats by religious groups have been in existence for many, many years and people have reported feeling refreshed and renewed in their faith and in their inner strength.
Time spent alone in a peaceful, natural setting is time rewarded by beauty and spiritual reflection. Yet, as Robert Frost writes, many people "have miles to go before... sleep." Individuals do have personal obligations to family and others that prevent them from contemplating beauty and communicating with oneself. Added to this, people tend to fall into the monotony of which Thoreau speaks--the beaten path blinds people to much that is around them. And, so, people must return from Nature for the same reasons that they retreat to Nature.
For me, starvation would be a definite disadvantage :) Seriously, I think it would be great to spend some time in solitude in a natural setting, but I think two years would be a bit much. It does depend on the meaning of "solitary." If it means totally alone all the time, this would be a bit much. If it means largely alone, but in contact with others as necessary, then I think it might work.
The main advantage would be to get away from all the superfulous "stuff" that has invaded our life, and to spend time confronting some of the essentials of life to, if I may paraphrase Thoreau, find out what's really there. Many of us today feel that we are overwhelmed by things that we have to do, but which aren't really all that important, and we spend too much of our energy/life pursuing things that don't really add to out lives, just our list of possessions. What would we find if we were separated from all these things?
I have done a 30 day solitary retreat, and can tell you that it was a very insightful and helpful experience ... but it's a bit short of 2 years.
Advantages: Many of the advantages are discussed in Thoreau's book "Walden". He feels closer to God, to nature, and learns just how much of the stuff of life is "frittered away by detail" and unecessary busyness. He learns the best thing in life is to "simplify, simplify, simplify". Being alone with nature would allow you to become truly in tune with yourself, to have time to ponder life, and ponder what is truly important.
Disadvantages: In my opinion, too much solitude is not a good thing. The human soul craves companionship, and sociality. To be alone for yourself for so long can often lead to emotional and mental illness (read "The Yellow Wallpaper" for a good example of this). To socialize and be loved and validated by others is a very important part of life, whether we want to admit it or not. Other risks are the possible dangers of being so isolated; if you are injured or hurt, it could be fatal (read "Into the Wild" for an example of this).
Though Thoreau was partially isolated for those two years, he still had many, many visitors, and went into town frequently. He had the best of both worlds, which allowed him, in my opinion, to find a balance that provided such profound and moving lessons. I think that we all need to find more alone time, especially in nature, but to take it to the extreme is hazardous to your emotional, mental, and physical safety.