I think there is a difference between stackers and collectors, when it come to grading coins.
In most cases it makes no sense to stackers to have there coins graded, as the buy them for the metal content not the collectors value of the coin itself. And most of there coins are made for this purpose, that's why the come it tubes and not in a fancy box.
For collectors it's different, but it also depends on what you collect. I mainly collect pre 1935 Dutch coins in mintstate condition, where MS64 is the lowest grade I want. I do grade all these coins with PCGS (sorry Numi) If I buy them raw, at coin shows or auctions. I do this to preserve there quality, so that in 50 years time another collector can still enjoy the beauty of these higher mintstate coins.
Adding value to these coins is not the main reason for me, but grading coins is getting more populair over here in The Netherlands. And the auction houses do sell more graded coins then 10 years ago. Whereby coins graded MS65 or higher do sell for way more then the catalog value, most of the times.
I think the most important benefit for these graded coins is, that you can now buy a coin from a foreign action unseen. And still know what the quality of the coin wil be, when it comes to wear and marks. If the photo's are really good, you can even judge the eye appeal without handeling the coin. I would never buy a ungraded higher value coin at auction, If I can not see it in person.
For modern collector coins it's difficult, because you don't know how populair the will be in 20 years time. Now people pay a high premium for a PF70 queens beast coin, because the series is still running. But in 20 years time the royal mint wil have produced an other 15 coin series and the queens beast coins might be forgotten by most collectors. I do grade some modern coins (also with Numi), but only to preserve the quality not to add value to them.