I don't understand why anyone would install any beta program.
Sometimes there are new and/or improved features in Beta program that a user may want to avail himself/herself earlier rather than later by waiting for the RTM version of that Beta. Of course there is a trade off on this. However, a user may feel that the tradeoff weighs in the Beta's favor.
I've read all of the posts on this topic and it looks like a lot of people have and are having a lot of problems with the Beta Version of Avast.
The choice whether to use a Beta version of a program is a game time decision. There is no clear cut answer and the choice either way may or may not be a correct one. For example if a Beta update offers security enhancements over the last RTM version one has to weigh the importance of those security enhancements with the possible problems that come with using a Beta. For example I was using W8-DP, W8-CP and W8-RP even though the previously called Metro part was far from usable. However even in the Alpha version of W8 the desktop part was usable and stable and there were sufficient security and memory usage enhancements in W8 Alpha to make it worth while to use especially since W8 even in Alpha was stable. What I did forgo was the use of some programs that would not run in W8 Alpha and some not even in W8 Beta. I switched back to W7 since going from W8-RP to W8-RTM (It will be available in two months) would have required me to install all my programs again whereas upgrading from W7 will not. So since I was going to have to reinstall all my programs either way I decided to go back to W7 for a couple of months until W8-RTM is available so I could get a clean computer prior to the upgrade. I did it last week because I will have too many things going on in October to spend the time reinstalling all my programs, whereas things are slow for me in August.
Also regarding the use of Beta and maybe more to your last comment, the quality of a software's Beta will vary over time, depending on among other things, the complexity of the upgrade and the quality of the staff in the company's software development area that designed and programmed the Beta.
Bottom line is there is no clear cut answer on whether to use Beta's or not. It is a game time decision. However, I will say this. If you are not an advanced Windows user and advanced user of the specific software I would recommend staying away from Beta's all together even if they offer security enhancements. Just keep all your software updated to to the latest RTM version. Major security flaws are usually available in minor software RTM updates