I think Double O said it best up top with "But every top tier MC that has lasted is one part lyricist." I was just looking at this list of best selling rap albums of all time:

http://www.complexmag.ca/music/2013/05/the-50-best-selling-rap-albums/

and who on that list wouldn't be considered a lyricist to some degree? I can only name 3 for sure non lyricists Master P, Nelly and Vanilla Ice. Arguments could be made for and against 50 Cent, MC Hammer, Beastie Boys, Salt N Pepa and Puffy (but a lot of people bought that album for Biggie). And don't say Will Smith isn't a lyricist, anyone who has heard He's the DJ, I'm the Rapper will tell you you're dead wrong.

Now singles wise things are a little different with all the Flo Rida's and Pitbull's and Tag Teams etc. but when it comes to long term careers and respect hip hop thrives on lyricism.

Patrick