I think I can understand the difference between having whiteness according to its full power and having whiteness according to something less than its full power, because, I take it, it is the difference between being less than perfectly white and being perfectly white. By contrast, there does not seem to be a difference between being perfectly existent and being less than perfectly existent. Existence is an on/off property: either you're there or you're not. Because existence is on/off, it would seem, either you have it according to its full power or you don't have it at all.
[Christopher Hughes, On a Complex Theory of a Simple God. Cornell (Ithaca, 1989) p. 27]
This particular version of the argument is directed at Aquinas; I'm setting aside the question of whether 'existence' as we usually understand it is an adequate translation of Aquinas's esse (I don't think it is, outside of certain narrow contexts; the notions overlap, of course, but there are a number of reasons to regard 'existence' as a term that fails to coincide very well with esse in meaning). Instead I want to point out that arguments like this seem to make an unwarranted leap. Suppose that X is an 'on/off' property; you either have it or you don't. It doesn't follow from this that you can't have degrees or gradations of X when it's 'on'. Why would it? The mere fact of there being no tertium quid between 'on' and 'off' tells us nothing about the nature of 'on'.
What people appear to be doing in these arguments is confusing 'degrees of X' with 'degrees between X and non-X'. The two are not the same, however similar the answers to questions about them may be in particular contexts. The first is a scale of X; the second is a scale between X and non-X, and these are two different scales. To have the former, you just need X to admit of intension and remission -- X's need to have some feature or characteristic that can be described by 'more' or 'less' in comparison with other X's. To have the latter, you need to have a characteristic or feature that X and non-X share in different intensities and or strengths. For the former, X can be a very precise sort of thing -- sharp boundaries between X and non-X -- without causing problems for the scale. For the latter, the line between X and non-X has to be fuzzy, vague, blurred. However similar they may seem, they really are extremely different.
So, even if existence cannot admit of degrees of 'fullness of existence' or 'perfection of existence qua existence' or what have you, the mere fact that things either exist or don't is not a good reason for thinking so. So it is with other things. To have degrees of truth, for instance, you don't have to admit that there are degrees between truth and falsity (likewise with degrees of falsity). Being committed to the claim that there are degrees of beauty is not enough to commit you to the claim that there are degrees between the beautiful and the non-beautiful. Commitment to the claim that there are degrees of goodness isn't a commitment to the claim that there are degrees between being good and not being good.