1) If you can't reduce glucose levels by other means then insulin is a life, sight and limb saver. ; Retinopathy, kidney disease, neuropathy are caused by high glucose levels not insulin. When someone hasn't enough insulin to stop glucose rising but does still make enough to stop DKA their levels can still rise high enough to cause coma .(fortunately fairly rare as it either happens when people don't realise that they have diabetes or it goes untreated for some reason)
http://www.diabetes.co.uk/diabetes-comp ... -coma.html
2) Not all people with type 2 diabetes make lots of insulin .
If someone hasn't been able to control their diabetes by all other means ie reducing resistance through exercise, through diet or a reduction of visceral fat then they probably don't make enough insulin for their needs.
They may produce lots but are extremely resistant to it .
They may have lost a lot of their insulin production over time and their beta cells no longer make enough.
They may have made less in the first place
They may not been able to increase the amount they make to counter their insulin resistance.
If other drugs don't work or are contraindicated for other reasons , they may absolutely need to take insulin to keep glucose levels down and avoid the problems mentioned in para1.
3) People in the intensive arm of the 10 year UKPDS study had HbA1cs ranging from 6.2%-8% . (some of the group took insulin and some sulphonylureas adjusted to achieve these results) In this group there was a 25% reduction in microvascular complications compared to those not treated intensively. There was a 12% reduction in risk for any diabetes related endpoint (from death to renal failure to amputation) There was no difference between the results for those on insulin and those on Sulfs . Unfortunately there was no reduction in risk for cardiovascular disease.
People taking insulin did put on weight; an average of 4kg. (ie not the huge amounts sometimes mentioned)
4) In contrast in one major recent trial (ACCORD) there was a higher death rate from CVD in those who were medicated to achieve an HbA1c of below 6%. This was thought perhaps to be due to higher doses of insulin. This now appears not to be the case, though the debate as to why this happened in this trial goes on.
http://www.medpagetoday.com/MeetingCoverage/ADA/40107
5) Those who are most insulin resistant can take many 100s of units of insulin a day, sometimes thousands. These people normally have genetic types of diabetes and without the insulin will have both high glucose and also little or no body fat. These syndromes often appear when young and if other treatments don't help will use it for life.
(heres a guy who seems to have one of these syndromes though it doesn't say if he needs insulin at the moment, as a cyclist he is very fit so that would reduce the need)
http://www.dailymail.co.uk/health/artic ... betes.html
There are others with similar conditions and they often require stronger U500 insulin.
http://care.diabetesjournals.org/conten ... 40.full#F5
Most people with T2 won't need to take anywhere near these amounts but it does show that if these amounts are needed, they can be taken . ( I don't know of any long term trials, the conditions are very rare)
6) Someone mentioned Beverley Allit, she killed people by injecting with a large dose of insulin causing hypoglycaemia. Hypos are indeed the biggest problem for most people injecting insulin. Fortunately most are not severe and we are unlikely to give ourselves this type of overdose deliberately.