Is it better to add roasted vegetables into beef stew?
You will get the roasty, caramelized flavors; how much influence they will have on the overall flavor of your stew will depend on several factors including:


How deeply you roast the them
How much you add, proportionately, into the stew
How strongly flavored the other items in the stew are


Roasting the vegetables will also cook them, so you will want to add them to the stew much later, so they don't overcook.

If you google roasted vegetable stew, you will find several recipes for inspiration.

As to whether this is a superior, or heartier flavor, that would be a matter of opinion.  You don't mention roasting celery or onions (partners with carrots in the classic mire poix), but these are also excellent candidates for roasting, as are all types of root vegetable.