Yup yup, very true. My main point, though, was that you can still use the 70-300 with natural light, as long as you have enough of it, and if you don't, you can always use a tripod, or bump up ISO (but not too much).
The trouble starts, though, when "enough natural light" doesn't have good character for portraits (e.g. bright, but overly harsh sunlight). The easy fix would be to just put the subject in the brightest shade possible, with a cheapo reflector or $2 white cardboard sheet from the store if you you're feeling fancy.