Well, enough people voted for it that you’ll find lots of people who will tell you it’s a good thing.
I know there are SAG-AFTRA members on the SDMB. Many of them are working actors and they’ll be able to give you a MUCH better answer than I can.
I am a member, but my career focus (or lack of focus) has been such that I’ve actually done very little professional union work. For me it’s mainly been my nightclub act, with a little bit of “Gee, it would be great to book some decent T.V. or Film!” tucked away in the back of my mind- but I’ve not really put the work into achieving “Working Actor” cred.
So, hopefully a dedicated professional will come along to expand upon my explanation and point out any misinformation I may have posted.
AFTRA was an open union and anyone who paid the fee to join was welcome to join. SAG had strict requirements that had to be met in order to join.
Professional actors who were SAG members generally found it in their interest to be AFTRA members as well (the two unions covered different areas of work). AFTRA members who did not qualify for SAG generally were always working toward qualifying then joining SAG once they met the requirements.
So for many people this has simplified everything greatly. They either had been members of both unions or they were members of one of the unions aspiring to also join the other. Now it’s all one union.
There was also a bit of a caste system with non-union actors on the lowest level, then AFTRA actors, then SAG actors. AFTRA actors who weren’t SAG were often treated as if they were non-union all together (in the eyes of agents, managers, etc.). Many actors, including the “top level” SAG actors never liked the caste system and preferred the “we’re all in this together” approach.
One possible “bad” aspect of the merger is that the new SAG-AFTRA union is a closed union with eligibility requirements the same as the old SAG requirements. I joined AFTRA when it was an open union. I just walked into their offices with a credit card and I walked out a full member. Booking work as an AFTRA actor was one of the recognized paths toward meeting SAG eligibility- a pretty nicely laid out path that I always thought was a good option to present to actors just trying to get a career started. Now there’s just one union and it is a closed union. Definitely makes it harder on the kid just stepping off the bus aspiring to make it in Hollywood.