Certain heroes like philto and wellby benefits a lot from consuming rally. While manually timing them is definitely an option, it is also quite a pain to time those.
sometimes the rally disappears just maybe 0.5-1s away and you might have thought the rally is going to last.
unless you can provide an option to easily see how long rally lasts, and what is the time left to cd, manually casting rally is also going to be a miss occasionally.
Hence suggestion for autotrigger of ult on rally heroes to be casted only when rally buff is up. I would prefer this as an option because some heroes may not really need to consume the rally buff and might just want to use the ult