A fraction of players use wheels that have clutch+H-shifter, even less so use it (it's slower), whereas everyone will feel a better tire model. I'd say it's more important than the suspension changes that everyone praises.
I'd like to see the numbers, but even so, if a player is not choosing to use their shifter, and it's "slower," it's because IT IS a terrible clutch model and a chore to use. In Forza 4, because the clutch model is good, in "lower-class" races (such as the Clio Cup) drivers who know how to use a manual transmission do have an advantage over those using sequential (stick, paddle, and controller). While sequential stick or paddle shifting offers (mostly) an advantage for more powerful and lighter cars with better handling (most super cars and race cars up through P1 and Formulas), a skilled driver can use the clutch, time their shifting, and heel-toe to better control street, sport, and some super cars, and they have an advantage over drivers in the same cars with paddle shifting. Though paddle shifting does have it's advantages on the higher end, it actually hinders a good driver on the lower to mid-range cars. I would definitely enjoy paddle shifting in a higher-end Ferrari, but I prefer a manual and clutch in something like a Lotus Exige.
So... a player that invests $300 to $800 in a wheel, pedals, and shifter (setting aside a $30-40 controller) because they want more immersion (and maybe bring their experience from real life), should not be upset about this? Gran Turismo 5 has been out for three years, there was an update to address this, and it made very little improvement, if any.
It was great I had other options I could turn to (Forza 4, PC sims), and use my wheel, pedals, and shifter so I could appreciate the major investment I made to have a "realistic" and more immersive experience in a simulator.
If I had ONLY bought a Playstation 3, my G27 (and now Fanatec setup) and Gran Turismo 5, I would be very upset, and Polyphony Digital, Sony, and Logitech would have heard from me, probably way before now. At the very least, I'm just disappointed--and I was a long time fan of the Gran Turismo series.
If GT5 was my only option after this investment, and I had heard there was an update coming to address this, and upon it's release there was very little improvement (as was the actual case when it did happen), I would have felt like I had been led on by Polyphony Digital, and they could care less about the investment I had made--and I would have sought some kind of compensation or replacement.
In the first year of "playing" Gran Turismo 5, I struggled with the clutch and completed it. Then Forza 4 came out, and their clutch model was what I had been waiting for. I might go back to GT5 occasionally to drive a paddle-shift or sequential stick car, but I can't stand a full manual transmission in GT5.
Considering my history with this series (I had a Japanese copy of the original Gran Turismo and was playing several months before most others were in the U.S.) I want to be "brought back" to continue to fully support everything Kaz Yamauchi and Polyphony Digital have delivered to it's long time fans.
Fix the clutch model, and I'll be back. Until then, I can't appreciate any other improvements Gran Turismo will offer.