The way that I have used and thought about aspec is as encompassing a lot of identities and issues that get brought up in the ace and aro communities (I am not agender and so do not have a close tie to that community, and can't speak to that side of things). For example, we often talk about different kinds of attraction, relationships, etc.
For folks I know who are questioning their identities and figuring out what labels might be right for them, the idea of "aspec" has been really helpful. It can be hard at first to completely tease apart what exactly is sexual and what is romantic attraction. Someone who is questioning their identity might find a community and some sense of identity with "aspec" even if they don't have very specific labels yet (or ever). Even for me, I have identified as aroace for ~6 years and sometimes still have a hard time parsing what parts of my experience would fall under aromanticism and what parts would fall under asexuality. For me sometimes they are impossible to distinguish between, but I know that they are "aspec" experiences.
The "aspec" term also allows for the overlaps in experiences of the ace and aro communities. While they are not exactly the same, there are a lot of things that our communities have in common, and a lot of people who identify with both.
The bottom line for me is that this term has been and continues to be very useful for a lot of people, and questioning the legitimacy and validity of the term stands to do more harm than good, especially for folks who are new to these communities.