I use a harness with a front clip, mainly because I have sighthounds who are particularly vulnerable to neck injuries, especially when they shoot off at break-neck speed with a lead attached to a collar. They also slip their collars very easily, so it's added security there as well.
Most dogs don't need a collar and will be quite happy to learn loose lead walking on a normal, flat collar.
The only two exceptions would be:
- If you are using one of the dreaded extending leads (they are the work of the devil imho) as they can lead to horrible neck injuries if used with a collar - same for long-lines and recall training.
- If your dog is a strong puller, who is taking their time learning to walk on a loose lead. In which case, a harness with a front clip (rather than a clip between the shoulder blades) can help as a training tool until they have learned a nice solid loose lead walk.
Nigella, reup and Juice, your pups are gorgeous. All these cute puppy pics are making me quite broody! 
Dh melted when he saw the picture of Alfie - we have a bit of a thing for scruffy beards in this house.
We are just back from the vets. Pip's leg is massively improved and the vet thinks he'll be fine to be off lead again by next weekend.
I'm quite proud of myself for taking both dogs to the vets at the same time with no mishaps. I usually book appointments with a short gap between them and just nip home to swap dogs inbetween, but we were at the main branch instead of the little one across the road today.