Is the 50-move rule ever used in grandmaster games? The 50-move rule states that if no capture or pawn move occurs for 50 consecutive moves, then either player may claim a draw. It seems like the 50-move rule is never truly necessary to determine that a game between two sufficiently competent human players is drawn. I know that there are forced checkmates that involve sequences of moves much longer than 50 (I was reading about a mate in 545), but none of these could be found and played by a human within any reasonable time control.
Do games between GMs ever actually end because of the 50-move rule, or do they always agree to a draw long before this is necessary?
Yes, sometimes the 50-move rule comes up in grandmaster play. One recent example that comes to mind was the game Ushenina-Girya (Geneva 2013), in which the reigning women's world champion failed to convert a knight-and-bishop checkmate within the required 50 moves, and the game was drawn. So this is a case where it's a theoretical win, but the stronger side couldn't put it away quickly enough. I would imagine there are some similar examples in the rook-and-bishop vs. rook ending.

[fen ""]
[Event "Women Grand Prix Geneva"]
[Site "Geneva SUI"]
[Date "2013.05.06"]
[EventDate "2013.05.03"]
[Round "4"]
[Result "1/2-1/2"]
[White "Anna Ushenina"]
[Black "Olga Girya"]
[ECO "D11"]
[WhiteElo "2491"]
[BlackElo "2463"]
[PlyCount "251"]

1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Qb3 dxc4 5. Qxc4 Bf5 6. g3 e6
7. Bg2 Nbd7 8. O-O Be7 9. e3 O-O 10. Rd1 Qc7 11. Nc3 Bg6
12. h3 Rad8 13. Qe2 e5 14. e4 exd4 15. Nxd4 Rfe8 16. Bf4 Qc8
17. Be3 Bb4 18. f3 Qb8 19. Bf2 Bh5 20. Qc2 Bd6 21. Nce2 Bg6
22. a3 h5 23. b4 Ne5 24. Qb3 Nxf3+ 25. Nxf3 Nxe4 26. Nh4 Nxf2
27. Kxf2 Bh7 28. Nf3 Re7 29. Ra2 Rde8 30. Rad2 Bc7 31. Nfd4
Bb6 32. Qf3 Qe5 33. Kg1 a5 34. Kh1 axb4 35. axb4 Qg5 36. h4
Qh6 37. Nf4 Re3 38. Qxh5 Rxg3 39. Qxh6 gxh6 40. Nh5 Rg6
41. Re2 Rxe2 42. Nxe2 Kf8 43. Bh3 Bc7 44. Bf5 Rd6 45. Rxd6
Bxd6 46. Bxh7 Be7 47. Nhf4 Bxh4 48. Bf5 Ke7 49. Nd3 Kd6
50. Nc3 Kc7 51. Kg2 Kb6 52. Kf3 Bf6 53. Ne4 Be7 54. Bc8 Kc7
55. Bg4 Kb6 56. Nec5 Kb5 57. Nxb7 Bxb4 58. Nxb4 Kxb4 59. Nd8
c5 60. Nxf7 c4 61. Ke2 Kc3 62. Kd1 Kb2 63. Nxh6 c3 64. Bf5 Kb3
65. Bc2+ Kb2 66. Nf5 Ka1 67. Ne3 Kb2 68. Nd5 Ka1 69. Ke2 Kb2
70. Kd3 Kc1 71. Ba4 Kb2 72. Nxc3 Ka1 73. Nd1 Ka2 74. Bc2 Ka1
75. Kc3 Ka2 76. Bb3+ Ka1 77. Ne3 Kb1 78. Nc2 Kc1 79. Ba2 Kd1
80. Nd4 Ke1 81. Kd3 Kf2 82. Bd5 Kg3 83. Ke3 Kg4 84. Be4 Kg5
85. Kf3 Kf6 86. Kf4 Kg7 87. Kg5 Kf7 88. Kf5 Kg7 89. Bd5 Kh6
90. Ne6 Kh7 91. Kf6 Kg8 92. Nf4+ Kh8 93. Be4 Kg8 94. Nh3 Kh8
95. Ng5 Kg8 96. Nf7 Kf8 97. Bh7 Ke8 98. Bf5 Kf8 99. Nh6 Ke8
100. Nf7 Kf8 101. Ne5 Kg8 102. Ng6 Kh7 103. Be6 Kh6 104. Bg8
Kh5 105. Ne5 Kh4 106. Kf5 Kg3 107. Bc4 Kf2 108. Kf4 Ke1
109. Ke3 Kd1 110. Bd3 Kc1 111. Nc4 Kd1 112. Nb6 Kc1 113. Na4
Kd1 114. Be4 Kc1 115. Bd3 Kd1 116. Nb2+ Kc1 117. Nc4 Kd1
118. Bg6 Kc1 119. Bf5 Kd1 120. Nb6 Kc1 121. Na4 Kd1 122. Nb2+
Kc1 123. Nc4 Kd1 124. Kd3 Kc1 125. Kc3 Kd1 126. Bd3 1/2-1/2