Allegory,
No supplement is really "mandatory". However, things like EFAs, and multi's in my opinion are important for optimal health as most people don't have diets w/ adequate amounts of these nutrients. Most people should take a multivitamin simply because most people's diets are deficient in fruits/veges and the micronutrients that come with them. I know I sure as hell could use more. Furthermore, taking supraphysiological doses of things like vitamin C impart anti-oxidant effects that have additional benefits. Remember RDAs are simply the minimal amounts of nutrients to thwart disease... To take in the 500-1000 mg of vitamin C I take per day I'd have to eat 10-12 oranges daily! Vitamins simply make it easier to get what you need in a more concentrated and convenient form.
In addition I think those of us who put a lot of demands on our bodies through exercise/stressful jobs, etc, require additional nutrients to simply supply the body w/ additional co-factors, etc, to cope w/ the added stresses. This is the reason I always recommend a potent multivitamin to nearly everybody.
ArcBound,
You're asking a great question. Unfortunately I don't have a simple answer for you. You were young and still a beginner when you made substantial gains on 125 g protein/day. In all fairness, the OP, at around 108 lbs could easily make substantial gains at around 125 g protein simply because he is a beginner, and young and his body will respond quickly to any stimuli.
For old farts like me who have been training for over a decade, a high protein intake is absolutely imperative as it helps to force a positive nitrogen balance within the body. This simply means that your body is taking in more amino acids (the building blocks of protein) than it is metabolizing and kicking out in the form of urea. Coupled w/ exercise and adequate nutritional intake a positive nitrogen balance (extra amino acids) gives your muscles adequate building blocks to induce hypertrophy and grow.
In addition, there are other benefits of a high protein diet for active individuals as protein has a potent thermic effect. This simply means that the very act of digesting/metabolizing protein will kick up your metabolic rate. Protein also increases satiety and can help you to avoid eating crap.
I'm around 187-188 lbs and I try and take in 250+ g of protein a day w/ 3500-4000 calories. The high protein intake helps me to stay lean, and I hope, pushes me towards a positive nitrogen balance. Back when I first started training I gained 30-40 lbs within a year and a half by simply eating normal food and training 2x/week. As an intermediate level trainee, after 12 years in the trenches, I simply cannot grow without this level of intake.