I often run into this issue and end up having to expire evars on visit which leads to our users complaining of complicated reporting.
Also a combination of 'expire on visit' or eventX would be very useful!
I appreciate this can be partially overcome by setting to a merch evar see my blog post about merch eVar's if you want more detail (http://www.arcana-imperii.co.uk/?p=47) but I do agree with Neil that there are scenarios where this doesn't help and in fact being able to expire by multiple events would be a massive help.
We would also like to be able to set more than one condition for evar expiration. For example expire of eventX or after 30 days (whichever comes first).
Flagging as a duplicate of Expire eVars on Both Time Period AND Success Event - show your support there!