Easiest way to understand this would be to equate your pigs to humans. Now if you're trying to get an Olympic sprinter for the 2012 Olympics from a 16 year old, you will have a professional nutritionalist who will devise the perfect diet, based on a varying training regime, with all the foods carefully measured and fed at exactly the right time to create the perfect 2012 athlete.
Similarly if you are running a 200 sow unit, and taking you weaners through to finishing, you will want to ensure that they are fed exactly the right amount at the right time, to both save money (not waste food) and ensure they get exactly the right type of feed at the right time so they grow the fastest. A week saved on each finished pig would save a £200K building, and £50K per year running costs.
Now a piglet will start by growing nervous system, then bone, then muscle then fat. So if you are trying to sprint finish your pigs in the fastest time to save money, you will feed weaner, grower and finisher to get exactly the right diet at the right time.
If you are simply raising a few pigs, then treat them like you should your under 8 children. I would guess that most of you don’t weigh your children’s food beyond the first born first year (first baby made of glass, second baby make of rubber as the saying goes), you get it roughly right, and judge from the results.
All pig foods will contain enough of the right nutrients to get a pig fully to maturity and then maintain it.
You will be choosing between lifestyle/belief choices of GM/non-GM - organic/non-organic and between quality of ingredients - Waitrose vs Aldi – no one thinks you will die from eating Aldi!
If you want to go down weaner/grower/finisher then I would not criticise, but it is not essential.
We feed all our pigs of whatever age on sow food.