Having the recently browsing and who's online blocks cached makes forums seem like there's no active members, even though the forum has many active members. Recent browsing block is the worsted On forums with few active members the impact of the blocks really is highlighted.
Invision forum for example i would always see people browsing but now its a rare to see many topics with people browsing due to caching, . If caching is to prevent performance hits then give the admin the choice to disable or enable the individual blocks. The caching of these blocks is all or nothing or reduce caching times in the present setup.