As other posters have said, "it all depends".
The basics: Buying on/off vs. full vig.
It only makes sense to buy on/off a number it it "hits" at 4.5% of the time. Now, how did I get that number? Well assume I go 500-500 ATS playing into 1/2 point lines with no buy. Obviously, I lose 50 units to -110 vig. If in 45 of those I lose by 1/2 a point, the buy will make my record 500-455-45. Now, I lose "only" 46 units, so my performance is ever so slighly better better.
The problem is that most games don't land on the number at 4.5% or higher a clip. The NFL 3 is a huge exception, teams favored by 3 win by 3 around 11.5% of the time! The NFL 7,10,14 and the College 3 and 7 all will hit at over 4.5% so in All those cases, the buy is the superior play to the nonbuy.
Obviously, when comparing a -105 bet at +x to a -110 bet at +x.5, now we only need to have the game "land" on the number at 2.25% of the time to prefer that extra 1/2 point. Virtually EVERY basketball side bet qualifies, a 1/2 point is worth more than 5 cents. On totals in hoops, it is pretty close proposition. In the NFL, most of the time the extra 1/2 point is preferred to the -105, but obviously obsurre number like 2, 5, and 9 are exceptions, as teams favored by 9 for example don't win by 9 at even a 2% clip.
Note, HUGE mistakes get made by even the experts in discussions like this. The biggest mistake I see is a WHOPPER, where guys say stuff like "16 percent of all NFL games land on 3", but forget that for the game to land on the number the favorite has to win!
The Double Whopper with Cheese is the mistake many experts make, where they conclude that buying a 1/2 point is a bad play as it still will result in a losing group of bets when played blindly. For example buying every NFL 6.5 point dog to +7, -120 is a losing strategy. However, the buy is the correct play GIVEN you are already going to play +6.5, -110. It is not correct to compare the buy to no bet, but rather to compare the bet with the buy, to how one would do when betting without the buy.
Note, in my analysis, I assumed a 50% ATS record, and 1/2 point lines. Feel free to use a 53.5% ATS record, and integer and 1/2 point lines. The math gets deeper, the results stay comparable.
As Wild Bill says, guys LOVE to make sweeping generalizations but one should do the actual math to figure out what is optima. The lazy man's method is to go to Pinny's drop down menus to compare buying selling half points, as they tend to charge just a little bit extra than you should pay to buy, and they give you just a little bit too less to sell half points.