For a lot of people, it is important because they want to keep using the bitcoin client that they have grown comfortable with, but with an additional layer of security that the trezor provides...
This is all true, but such discussion belongs to the the forum of respective software users like. I believe that the Trezor devs did provide Pi shields to Multibit devs one year ago and they still decided to postpone the support to MBHD which seems to never happen. There is not much more we can solve here... The device is out there and if Multibit still does not want to provide support for thousands of users who have it, they will loose those users.
And frankly, if you have BIP32 implemented (which I highly recomment for *all* wallets) then supporting Trezor is not that hard.
IMO, the trezor web wallet has enough functionality, to not need another BTC client along with it. You can see the balances without needing to log into anything, and you can send coins with ease. I guess if you want to be able to send BTC from your mobile device, you should also have a blockchain.info wallet or coinbase or something like that, but as far as savings go, you don't really need anything beyond the trezor wallet...
For smartphone spending, I use Mycelium, which is quite well thought out. For saving, I love my Trezor as is (with mytrezor.com), and will absolutely buy another one as a backup. However, I still think I need some serious time and regular use to feel comfortable to get rid of paper wallets, and go all-in with the Trezor. I can see that happening, but just not yet. With paper wallets, I know that I am in complete control, and that gives me comfort.