I guess I can shed a little more light on the issue.
I believe its because of sense UI. we are not able to remove defaults from our messages app. its grayed out.
Unfortunately, I can’t clear default privileges for HTC’s messaging app. I would love to have Forever Alone open GoSMS, but without making it the default program there’s no way
I have HTC Desire and I use Handcent SMS. I downloaded the widget and I love it but it opens android messaging when I have a new sms. I tried to follow the instructions above but it still isn’t opening handcent when I get a text.
I got it working on my HTC Desire (rooted) BUT mms won’t work so I gave up. Too bad.. this widget is cool. lol
If anyone wants to know how:
Install titanium backup and freeze the default messenger app.