I don't think he used it any less in 2009/2010 than other times (except before he used it a lot). The thing with his drop shot seems more to be how well he's picking it as a show choice.
When he played this year's Aussie Open he was doing it really well on the whole - and then against Murray he barely won a point when he tried it. It's not just that Murray is some super fast legend - rather that Federer was pressed to try something and was hitting them at inopportune moments more often than usual.
Original Pro Staff 85, leaded to 370g, hybrid poly/syn gut set-up, 48-52-ish lbs.