It's not feasible for Twitter to evaluate what person actually typed the characters, and that isn't very relevant to the issue of knowing whether the message is something the person supports anyway. The check-mark means the account is verified to be owned by a certain person, and I think it's very useful to know whether an account is owned by a certain figure/politician. It's very useful to know that this account is making posts supported by Mike Gravel, and that these kids are not just people he's never met making posts under his name. Most popular politicians have a lot of popular parody accounts, and knowing whether you're looking at a message made by a campaign vs one made by someone parodying a campaign is valuable information. At the end of the day, if an account is making posts [verified person] does not support that person can either (a) change their password or (b) ask twitter to remove the verification. The blue checkmark then signifies the account is actually run by that person, regardless of who is typing the majority of the posts. It's a matter of common sense that it's possible for me and anyone else with an online account to give my password to someone else, and then that person could make posts with my account. What the check mark tells me is that no one has taken the account and locked the owner out of it, because if they did the person would have just told Twitter and the check mark would be gone.