Is the base game worth playing if I have the option to play with expansions?

Personally, even when just playing with the base game board, I always add some of the expansions' game elements, like madness and injury cards, epic battle cards for the ancient ones and the like.

Playing with none of them during the summer league felt kind of weird.

Besides, a game without gate bursts, only the arkham locations to open and no mythos cards that add 2 doom tokens to the doom track, the game suddenly feels too easy.

It's worth it if you're teaching a newbie, or ARE a newbie; or maybe for sentimentality after you've played a bunch of games with expansions. Or for League play.

But otherwise, no I'd always wish to play with at least one board and at least one card expansion. When Arkham Horror first hit BGG, it was in the 90's in ranking. Now it's climbed all the way to 51/52! This is due in no small part to the Dunwich and later expansions.

We do it for all the reasons Tibs mentioned and like it a lot. Especially after playing difficult scenarios or multiple expansions again and again, playing just the base game is great fun for us. Compared to scenarios that feel more like work sometimes, it's a great way to spend a relaxed evening with friends. Much quicker to play and much easier on the brains