It doesn't restrict wifi capability, as far as I understand, as we have wifi tether w/out replacing the kernel on the D2...what is important (from my experience loading kernels on D1) is that your kernel and the associated wifi module are matched to work together.
With kernel updates you can build functionality - such as support for open vpn, into kernels, but you can also do that w/a seprate loadable module w/out touching the kernel (for example, here's a tun.ko for the DX, which also has a locked bootloader - attached is a functioning tun.ko for droid x - xda-developers).
So there may be some additional convenience w/being able to load custom kernels (built-in modules) but I haven't heard any dev identify the lack of any options other than overclocking itself.
Above is based on what I've read in general, and discussions I've watched between devs on the #koush IRC channel.