Easy to abuse for both parties.
If you don't like it, you are free to accept another job offer or to quit and make your own business out of that thing you were thinking of but didn't do because you feared your employer would claim ownership.
Here is a Route 128 story: back then in the mid-80s Elias Corey employed Stewart Rubenstein to work on LHASA, his retrosynthesis program. Remember that Corey was awarded the Nobel not for total synthesis but for automation of retrosynthesis. Rubenstein was approached by a colleague from David Evans' group of he could also automate structure drawing. This was the beginning of Chemdraw.
Corey (it's well known that he is an arsehole) claimed the Chemdraw IP because he paid Rubenstein's salary and Evans supported him in the suit against Corey. (Evans is an arsehole as well, but a different kind.) Rubenstein won the suit because he could prove that off-the-clock he worked on a Macintosh and on the clock presumably on a PDP-11 or somesuch.
Should the Coreys of Massachusetts be able to claim IP unrelated to employment because they pay your salary and own you 24/7?
This is a thread where people try to understand why a state maintains such boneheaded policies which are actively driving it into the ground. It's not a forum for people complaining about the policies' impact on them, personally.
So, re: "take your ball and go home": everyone already did.