There are a lot of things that bother me about the logic of the charge for people holding visas of more than 6 mos.
The purported logic behind implementing this charge is to not allow those who are not contributing to the UK (presumably via income tax, since that is what funds the NHS) to use the NHS.
I can somewhat understand this for Tier 4 holders, since although they are allowed to work, it is only 20 hours/week. However, Tier 4 holders will also be contributing hugely to the UK economy by virtue of the fact that they are paying international fees to their university (i'm paying nearly £14k for my masters). Like I said though, as this is not direct tax contribution, the £150 charge seems reasonable, given that it is far far less than I'd pay for my healthcare here in the US.
However, past Tier 4, the rest of this really doesn't line up with "contribution to the UK" reasoning. Firstly, for most PBS migrants, they are there specifically to work, so they are guaranteed to be holding a job that pays a certain amount and are thereby guaranteed to be contributing a minimum amount of tax. So although the fee (£200) is pretty much nominal, it hardly seems fair or inline with the "contribution to the UK" reasoning to effectively charge them twice when they are already present as highly skilled workers who are paying taxes.
As for FLR or spouse visa holders, they are pretty clearly showing a commitment to the UK by having a settlement visa in the first place (not to mention absolutely exorbitant cost of obtaining that visa). But even beyond that, if the real reason for the minimum income requirement for those visas is keep spouses from becoming scroungers, then giving them NHS without an extra charge is the only logical thing that follows. Because the UK government has already determined an income level at which we are not a drain on the UK economy/taxpayer. On top of that, many, if not most, FLR and spousal visa holders will have a job and be paying tax personally.
I'm not debating the fact that this law was passed, or the amount of these charges really, just the premise upon which they are being implemented.