Just because a book said it was preferred in 1848 to use he when sex is indeterminate (then went on to use both singular he and singular they) doesn't mean that's true today.
The language always used both. Why stop today? Might feel a bit odd at first if you're not used to it, but it's not a bit different than how singular "you" is used ("You are coming today?" uses the plural form of "be").
The book itself used both forms! Shows you that English teachers may have been overly strict with you in the past.
Some more on the topic:
Anyway, "They" has been undergoing this process for a few centuries now, and I concur that it's time to embrace it.
I don't really think it's an evolution, as much as the mid1800's tendency to over classify and striate things going away.
It is one of those niggles that irks my pedantic self though; particularly the politically correct change to using 'she' instead of 'he' (instead of the perfectly neutral 'they', and even 'one').
I've always used "he/she". But that's not quite accurate (or elegant) either.
Having said that, fields with a more even gender ratio have had to deal with this for a while - what do they do?