According to O'Brien, Winston was followed and closely watched for about 7 years, before their meeting took place. In a society like the one depicted in the book, where people from all walks of life are being "evaporated" on a daily basis — in most instances "just-in-case" and not for actual crimes — I can't understand why a man like Winston, who clearly committed severe thought-crimes (his diary, as the most extreme example), shouldn't be just "evaporated"?
What was the point in the secret meeting, the indoctrination, in passing him a copy of "The Book", in giving him time to read it, and only then arresting him? The only thing I can think of is that the Thought-Police wanted to try and take as many people as possible when they arrest Winston, as his "followers" or something of this sort; but after 7 years of close surveillance, they knew he was no leader of some resistance group. It was just himself, and then himself and Julia, and that's it.
Why give him time and risk him telling someone else about O'Brien, and by that potentially "blowing" his cover? (Such an act might cause a "snowball" effect, where some people might spread rumors about O'Brien, while others might actually talk to the Thought-Police.)
Moreover, what was O'Brien's point in writing "The Book", which actually had much truth in it? What was the motivation for doing such extreme amounts of work, which involve enormous "amount" of Double-Think, instead of just arresting and shooting Winston? Nothing would've changed if the book didn't actually exist at all.
And finally, what was the point in holding him for so long in the Ministry Of Love? What was the value in spending so much time and effort in breaking one individual, which has no social footprint whatsoever? He's no leader of an underground resistance group; Winston is actually, as far as the Party's concerned, no one.
The only reasonable option here, is that the Ministry Of Love employees, just like Winston in his new job, need to show for their supervisors that they actually do the job intended by the Ministry of Love, and they're actually successful in that. That Winston, as quite an extreme case, was also finally broken.