These versions are not considered stable. So, using them is risky. The probability of getting into trouble is bigger, and this is the reason Not to use them.
The reasons to use them are:
- You start using more recent software earlier. So, in most cases you will enjoy a more modern software ecosystem.
- You get the possibility of helping the project better, since you are then able to report problems with the Alpha, Beta and RC versions.
As for me, given that I am a programmer and that I am highly dependent of my computer to work, I always use the Stable Release. Though, in my private computer I enable the proposed and backports repositories, so I am able use the most recent (and unstable) software (for which I can report problems too). At my job's workstation I only use the regular repositories.
While I was a student I always used the beta release, and it was once problematic when trying to upgrade it.
edit: There is a third option: You can also stick to the LTS releases and just update LTS releases. This is the least-maintenance-requiring option.