Surely its a personal decision, some use an agent, pay for a service and get peace of mind, don't go to immigration and have to deal with sometimes difficult officers?
Some folks decide to go to immigration with their paperwork and take their chance?
Now most folk that do the latter have everything in order and things go well, but I have seen reports on AN where they haven't, possible ill prepared paperwork or a difficult officer, I don't know? If you live close to the IO, its usually not a problem, but some folks do live 4 hours plus away, then its a problem, if you need to get something?
Not all IO's are the same, there is inconsistency not just in the individual offices, but even individual officers?