@Derek ******
if you have a visa, when you arrive at passport control, technically your saying 'i've been given permission by your foreign office to enter the country, if everything is OK, can I enter'?
While a visa exemption is saying ' please can you check if i am able to enter the country'.
With a visa the majority of the IM's job is done for them, and its pretty much just a stamping exercise. Without a visa your 100% in the hands of the IM to look at your travel history and for them to decide wether you fit the tourist criteria.