No. They (over) charge for beer. So, No reason EXCEPT if it's your dream to walk on the warning track or sit in the stands. On the other hand, I believe I read they're giving out AG awards or, at least, announcing the top 3 in each AG (male and female) at Fenway. Have to double check on BAA site but I thought I saw it there. So if that's in the cards for you
(you're in the top 3), head on over, I presume. But double check they're doing that.
Nevertheless, it's not worth it especially if the weather's not good. Go out and have a great dinner WITH your medal on. Bostonians are great and appreciate any runner that has finished the Boston Marathon.