I think Franklyn Stephenson was the first bowler I heard of with a specialist slower ball, and I think that was probably a knuckle ball. Chris Cairns developed a very good slower ball in the 90s, prob also a knuckle ball.
Essentially, the more batsmen are looking to generate bat speed and the more damaging dot balls are, the more dangerous off speed deliveries are. For a primarily defensive batsman in long form cricket, a bunch of off speed pitches probably aren't going to do a lot - a dead bat defensive shot, by the nature of it, largely negates the issue of timing because you're just moving the bat into line and then waiting for the impact. You can be early on the shot with little to no penalty.
As soon as you swing hard, you're reducing the amount of time that the bat spends passing through the ideal interception point so coordinating your swing so that bat meets ball at the most powerful point of the arc becomes increasingly difficult.