I see this a lot, working at a HS and having a CP who does allstar. I think they can benefit each other well. Allstar makes HS cheer better, IMHO. Those girls are in the gym all the time, and practice makes perfect, right? I have seen both AS coaches who frown on their girls cheering HS (mainly level 5 girls who might get hurt with less "experienced" HS teammates), and I have definitely seen HS coaches who think AS is bad (taking time away, trashy, dangerous, I have heard them all). They can co-exist. Gyms also stand to make a lot of money by opening up mat hours for HS teams to use and get better. The AS gym may pick up some more girls that way, and at the very least, they are collecting "rent" from the teams that use it... but that only works if there are HS teams who compete, not just sideline cheer. Look up the pictures of ACX's BEAUTIFUL gym... they finance a lot of that by having HS teams use their facility.
Some of the best NC HS teams are ones closely associated with AS gyms nearby. One local HS for us has a bunch (I believe it is 6 or 7) of girls from CEA CoEd Elite and Senior Elite on one HS squad.... including a couple of "Cheerlebrities". It is nice to see them OUT of that role, even though I have nothing against the Cheerlebrity phenomenon.... they just get to cheer and have fun, and not worry about crowds when they leave.