What is the relative importance of cut-retardant safety gloves when using a chainsaw?
A diagram put out by the US Product Safety Comission shows that the left leg and left arm/hand are the most commonly parts of the body injured by chainsaws:



(Source: OSHA Web site)

If the dots on the diagram represent frequency of injury,  protecting the left hand would help prevent a common source of injury.

The State Compensation Insurance Fund website also states that "The most frequent chainsaw injuries occur to the left leg and the back of the left hand."

The diagram on the OSHA site dates back to 1994, so it's not clear if modern safety features like chain brakes have changed the distribution of injuries.

Note that the  STIHL Cut-Retardant Gloves referenced by the asker of this question only provide cut protective material on the left glove: "Engtex®, a cut-retardant material, is sewn into the back of the left glove", so Stihl too has identified this as a particularly vulnerable part of the body. 

To be honest, from personal experience, I'm unsure why the back of the left hand is so frequently injured, it must be because that hand can slip off the grab-bar when the chainsaw kicks back and the hand slides into the moving chain.

Since I was curious whether or not the current injury distribution matched the 1994 statistics, I did a little more research and found a CPSC query tool that gives access to their injury data. 

When running this tool against their 2012 data, it reported that the CPSC estimates that there were 30,509 chainsaw injuries, and they had detailed data for 635 cases. I downloaded the detailed case data and counted up the number of hand, wrist and finger injuries:

Hand:   54  injuries (8.5% of total)
Wrist:  15  injuries (2.4% of total)
Finger: 123 injuries (19% of total)


They also provide limited details of each accident, here is limited descriptive data for a random 15 hand injuries:


59YOM USING A CHAIN SAW TO CUT BRANCHES CHAIN KICKED ..LAC TO HAND
40YOM-HAND LAC-GOT CUT WHILE USING A GRINDER TO SHARPEN A CHAIN SAW CHAIN-@ HOME
PT'S FRIEND WAS TRIMMING BUSHES WITH A CHAIN SAW AND PT'S HAND GOT IN THE WAY. LACERATION HAND, RIGHT. 54 YOM*
CUT HAND ON CHAIN SAW. LACERATION HAND, COMPLICATED. 44 YOM* 
59YOM CUT HAND WITH CHAIN SAW/ RT HAND LAC   
43YOM THAT WAS HELPING A FRIEND CUT DOWN A TREE WHEN HE WAS HIT IN   THE RT HAND WITH A CHAINSAW. DX: RT HAND LACERATION
17YOWM WHILE HOLDING CHAINSAW IT SUDDENLY FLIPPED &amp; JERKED HIS HAND VIA      A ROTATIONAL FORCE. DX: SPRAIN R HAND
32 YOM CUT HAND ON A CHAIN SAW AT FRIENDS  HOUSE     DX/ LEFT HAND LAC
46YM CUTTING WOOD WITH CHAIN SAW THAT KICKED BACK CUTTING HAND       >>LAC
88YOM HELPING SON CUT A TREE, PATIENT WAS HOLDING TREE LIMB, CHAIN SAW       ACCIDENTALLY CUT HIS HAND .  DX.  LAC. HAND.
57YM CUTTING A TREE WITH CHAIN SAW WHICH KICKED BACK CUTTING HAND    >>LAC
54YOM WITH LAC TO HAND FROM A CHAIN SAW
49YOM CUT HAND WHILE USING CHAIN SAW DX: HAND LACERATION     
49YMC CUTTING A TREE WITH CHAINSAW KICKED BACK FELL CUTTING PALM ON CHAINSAW/LACERATION
17YOM LACERATION TO HAND CUT WHEN THE CHAIN SAW HE WAS USING SLIPPED CATCHING HIS GLOVE


Based on this data, it does appear that kickback is a significant factor in hand injuries -- I didn't look at all of the records, but looks like kickback is cited around half the time for hand injuries.

When looking at finger injuries, kickback doesn't appear to be listed as a cause as often:


41 YO M PT WAS USING A CHAIN SAW AND CUT THE TIP OF HIS LT MIDDLE FINGE
35 YOM INJURED FINGER WITH CHAIN SAW. DX-LACERATION RIGHT INDEX
20YOM CUT THUMB ON CHAINSAW/ LT THUMB LAC WITH TENDON INVOLVEMENT
FINGER LAC, 37 YOM CUT FINGER WHEN USING A CHAINSAW
41 YOM LACERATED LEFT THUMB ON A CHAIN SAW
49YOM CUT HIS HAND ON CHAINSAW. +ETOH/LAC FINGERS
38YOM CUT FINGER WHILE USING CHAIN SAW DX: FINGER LACERATION
44 YOM SUS LAC TO FINGER, ACCIDENTALLY SLID CHAIN SAW OVER FINGER
LT THUMB LAC.34YOM LACERATED THUMB ON A CHAIN SAW.
30 YOM SUS LAC TO FINGER, HIT IT AGAINST JAGGED PORTION OF CHAIN SAW
71 Y/O MALE CUT FINGER WITH CHAINSAW AT HOME
55YOM CUTTING SOME BRANCHES USING A CHAIN SAW INJURED LT FINGER;
58 YOM W / LACERATION TO RT 3RD FINGER S/P WAS HELPING SISTER CLEAR UP
55YOM  PT CUT L INDEX FINGR W/ CHAINSAW AT HOME.  WAS NOT WEARING WORK
45YOM LACERATION TO THUMB CUT WHEN CHAIN SAW HE WAS USING SLIPPED


For comparison, I looked at the distribution of injuries for 1997 (the earliest date they provide downloadable case data for), and the distribution is similar:

Hand:   57  injuries (10% of total)
Wrist:  9  injuries (1.6% of total)
Finger: 145 injuries (25% of total)


I'm not sure what conclusion to make from the data -- if the gloves provide protection for the hands and fingers, they appear to be a good investment and will protect you from a common source of injury. If they are only effective for hand/wrist injuries, those injuries only account for around 10% of all reported chainsaw injuries, so there may be less value in the gloves.