Why is Clint Frazier rated 34th best prospect by fangraphs.com? How valuable are "prospects"?

fangraphs.com
2017 Top 100 Prospects by Eric Longenhagen - March 13, 2017

34. Clint Frazier, OF, New York Yankees

Drafted: 1st Round, 2013 from Loganville HS (GA)

Age	22	Height	5’11	Weight	185	Bat/Throw	R/R

Tool Grades (Present/Future)

Hit	Raw Power	Game Power	Run	Fielding	Throw
40/50	60/60	50/60	50/50	45/50	60/60

Scouting Summary
Elite bat speed has long been the calling card for Frazier, who has hit and hit for power all the way up the minor-league ladder and whose batted-ball profile should play favorably in Yankee Stadium.

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Year	Age	AgeDif	Tm	Lg	Lev	Aff	G	PA	AB	R	H	2B	3B	HR	RBI	SB	CS	BB	SO	BA	OBP	SLG	OPS	TB	GDP	HBP	SH	SF	IBB
2013	18	-1.7	Indians	ARIZ	Rk	CLE	44	196	172	32	51	11	5	5	28	3	2	17	61	.297	.362	.506	.868	87	1	3	0	4	0
2014	19	-2.5	Lake County	MIDW	A	CLE	120	542	474	70	126	18	6	13	50	12	6	56	161	.266	.349	.411	.761	195	6	7	1	4	1
2015	20	-2.7	Lynchburg	CARL	A+	CLE	133	588	501	88	143	36	3	16	72	15	7	68	125	.285	.377	.465	.842	233	4	9	5	5	2
2015	20	-2.2	Scottsdale	AZFL	Fal		22	98	89	15	25	1	2	3	8	4	3	7	27	.281	.347	.438	.785	39	1	2	0	0	0
2016	21	-3.9	3 Teams	2 Lgs	AA-AAA	CLE-NYY	119	520	463	75	122	27	5	16	55	13	4	48	122	.263	.335	.447	.782	207	8	3	3	3	0
2016	21	-5.5	Scranton/Wilkes-Barre	IL	AAA	NYY	25	108	101	17	23	2	3	3	7	0	0	7	30	.228	.278	.396	.674	40	1	0	0	0	0
2016	21	-5.5	Columbus	IL	AAA	CLE	5	21	21	2	5	0	1	0	0	0	0	0	6	.238	.238	.333	.571	7	0	0	0	0	0
2016	21	-3.3	Akron	EL	AA	CLE	89	391	341	56	94	25	1	13	48	13	4	41	86	.276	.356	.469	.825	160	7	3	3	3	0
Minors (4 seasons)					Minors		416	1846	1610	265	442	92	19	50	205	43	19	189	469	.275	.355	.448	.804	722	19	22	9	16	3
All Levels (4 Seasons)							438	1944	1699	280	467	93	21	53	213	47	22	196	496	.275	.355	.448	.803	761	20	24	9	16	3
AAA (1 season)					Minors		30	129	122	19	28	2	4	3	7	0	0	7	36	.230	.271	.385	.657	47	1	0	0	0	0

OPS .657 at AAA is hitting all the way up the ladder?

50 homers in 1,610 AB is hitting for power? 3 homers in 122 AAA AB is hitting for power all the way up the ladder?

Excluding rookie and fall league ball, it boils down to:
2014 A: 542 PA, .761 OPS
2015 A+: 588 PA, .842 OPS
2016 AA: 391 PA, .825 OPS

Nice progression, right? It should be. The guy was drafted number five overall in 2013. NUMBER FIVE. It took him three years to reach AAA. THREE YEARS.

What the heck kind of prodigy is that? And Yankee general manager Brian Cashman stated before spring training that he expected Frazier to spend 2017 in AAA. That would mean that Frazier will make the Yankees no earlier than 2018 after being a pro four and a half years. FOUR AND A HALF YEARS.

That doesn't sound like anyone special. And Frazier is rated 34th best prospect by fangraphs.com?

Yanks tank: trade Chapman and now Miller for unproven minor league players. Sunday, July 31, 2016

Electric bat and dirty uniform: how Brian Cashman described his prize acquisition, Clint Frazier. Monday, August 1, 2016

Yankees from first round 2013 draft: Clint Frazier, Billy McKinney, Eric Jagielo, Aaron Judge, Ian Clarkin. Monday, August 1, 2016

Alex Bregman for Aroldis Chapman and Andrew Miller? Why didn't Brian Cashman bundle them? Monday, August 1, 2016

fangraphs rank of Brian Cashman's Magnificent Seven minor league players acquired for Chapman and Miller. Monday, August 1, 2016

Bozo look-alike contest: Yanks Clint Frazier is a 2017 contender ... if he makes the team. Monday, February 27, 2017