tuning the strings against each other is easy enough but if you play with other musicians [ piano , brass , woodwind ] it is handy if you get an accurate reference point
having a guitar that is perfectly in tune with no one else makes you look bad [ dont ask me how i know this ]
Its easy to get a reference point if you have perfect pitch , besides, its just as easy to ask one of the other muso's for an "E" and tune the whole guitar, takes less than a minute....

I have a good friend that has played and bass with me for years, when i lived up north {about 600K's away} Danny would ring me and i'd tune his guitar over the phone, yes he thought {and probably still does}that i was a smart arse.......

Actually what makes that easy for me is playing with CD's and records all my life, records were worse because most record players were slightly out of tempo, making it a must to tune up every time you changed a record or played somewhere else, well almost anyway, CD's are much better but a lot of bands i listen to use different tunings or tune the whole guitar down a half or full step, anyway i solved the constant tuning problem by buying a pile of guitars and tuning them all differently...

What guitars do you have Simon..?