How important that is to them (and to how much of them)?

Well, if you're going to a 53 degree orbit, you're in luck. But if you have a specific polar orbit, well... You'll need a special transfer vehicle package (SpaceX is partnering with Momentus), which will increase the mass you need to launch and has its own costs, making it more comparable to RocketLab.

RocketLab will let you go to whichever orbit you choose.

Also, RocketLab is pursuing reusability as well, so that may allow RocketLab to compete more directly on cost. Also, RocketLab can launch smaller payloads (in bundles with other customers) for less than the $1 million minimum SpaceX has.

Still, I think SpaceX has a sizable advantage for a lot of launches.

IF you have a payload that needs a non-standard orbit you absolutely need it to make your project work at all. Problem is that there are 50+ small launchers trying to make it and with the big volume going SpaceX its hard to see how the economics works out for that.

The market size is growing, but not as fast as some people hopped.