If you bowl to the batsman's strengths, even if you set a field to cover this, the batsman will still score freely, probably.
For example, if a batsman has 'strong offside preference', if you bowl on offstump and put most of your fielders on the offside, I have found that the batsman will probably whip you through the legside.
However, if a batsman is 'strong legside preference', if you do the same thing and bowl offstump with a mainly offside field, strangely, the batsman is less likely to whip you through legside, I think.
Funny that. You would think it's the other way round, wouldn't you?
A reason could be that, if you bowl to a batsman's favourite line, it gives him confidence to play strokes all around the wicket, regardless of whether some of these shots are against a batsman's preference (i.e. leg glance for an offside preference batsman).
Of course these observations I've made could be totally wrong. The quality of the batsman could have a major impact as well.
One thing I would suggest is to experiment by picking a field which is different to the line you are bowling to, i.e. picking an offside field, while bowling at middle stump, to an offside preference batsman.
But personally, I always tend to just bowl to a batsman's weakest line, and set a field which matches the line (i.e. bowl at leg stump, with a mainly legside field, to a batsman who prefers the offside)
