This year, if the team has a similar finish to things (i.e. out in 4-5 games in the first round), while players from the 2014 and 2015 drafts (1/3 of the roster) combine for less than 12% of the team's total minutes played, that would be frustrating to me.
I agree that if we were to lose in a sweep (or near sweep) in the first round, that would be frustrating.
As to the other part of your complaint, I feel like ideally you would want your end of the bench guys to be first or second year late draft picks. If you can develop them into long-term useful players, awesome job!! If not, you can replace them without too much lost. Nobody's going to cry too much if your seventeenth or twenty-seventh pick doesn't pan out.
I mean, it beats having veterans who cost more and have no upside on the end of your bench.
I guess my feeling is, unless you've got a mandate to win in the short term because of the age of your core players, or because you've got a shot at a title, I don't see the point in drafting guys only to sit their butts on the bench all year long.
Draft picks have a lot of value as trade pieces, and they have a lot of value when they provide cheap rotation talent. When you use a draft pick on a guy and then park him on your bench, you're just wasting a roster spot. You're not developing the player and you're not getting any depth in the case of injuries (e.g. Crowder's out and now Smart is our starting SF).
You're right that some draft picks just aren't rotation-caliber NBA players, at least not during the life of their rookie contract. There's value in finding out whether or not a guy you draft can play, and the Celts haven't really done that this year either. Some guys are too young and raw when drafted to have a real chance at contributing their first couple of years. That's fine. Ainge drafted three 21 year olds last year, though, and none of them has really played this year.