Cyclists are not some evil malicious group intent on making things worse for you. If they’re on the road, it’s for a reason. There are numerous possible reasons, and it’s not up to you to decide what that is. Just chill out and drive safely. Some of those reasons can include:

• Bike lanes are often blocked by temporary signs, parked cars, etc.
• Bike lanes often get debris like gravel, sticks, and leaves swept into them.
• Separated bike paths might not have convenient entrances that let the cyclist get on, forcing them to continue along the road.
• Separated bike paths might not have convenient exits where the cyclist is headed.
• Separated bike paths might not be well-designed to keep cyclists safe and ensure they have priority at driveways and crossing side-streets.
• Paths might literally just not go where the cyclist is headed (e.g. they continue straight, cyclist needs to turn right, which can only be done/is only reasonably quick when done from the general lane).
• Paths might be disconnected or temporary, and not worth getting onto in the first place.

Some of these might be, as you suggest, the result of them being “poorly planned”, but it can also be the inevitable consequence of historical bad planning even if the specific path here is good.

Looking at what I think is Wimborne Rd on Google Maps (because I don’t know the area, and what Google tells me is Wimborne Rd is clearly not what you mean, I’m assuming you mean what Google calls Hartcliffe Way or A3029), I note a few things:

• They appear to be a shared paths, not dedicated bike paths. If there are a lot of pedestrians around at a certain time, it can be a bad idea to use them. This may especially be an issue as it looks like a significant bus route.
• Street furniture galore. Lights, bus shelters, even an awful chicane at one point.
• Leaves.
• The path is not continuous across all driveways and side streets, including the one across the side-street Google calls Wimborne, and an extremely dangerous looking one crossing Headley Ln.
• You say it’s on both sides, and for a stretch that’s true, but the eastern side becomes extremely difficult to navigate (if it even exists) north of the turnoff to Parson St, and the western side disappears entirely south of Novers Ln, necessitating a multiple-stage crossing to get onto the eastern side if you want to continue.