Playing devil's advocate, the positive of allowing legislation to include unrelated riders is that it promotes compromise. And compromise is how a healthy democracy should operate.
Allowing several issues to be passed as a singular unit provides opportunity for an agreement to be made about several issues at once. Think of it like a Collective Bargaining Agreement.
Or if that's really impossible, you could compromise on separate bills. If people ever break promises, that's a reason not to trust them in the future and it's a lot more clear to the public about who voted which way rather than having a rider which no one really understands where it came from.