Hey, I've spoken to quite a lot of course directors from Bournemouth Arts, LCF, LCC, Newport and UCCA and they have all said that the reason they recommend taking a foundation year is that it gives you that extra level of maturity that you will need to bring to your work. Along with this, many people change their minds on degree courses during a foundation year as it gives you the oppertunity to try out many more art disciplines than you possibly could have done at A-Level. Although, in saying this there are cases where A-level students have gone happily straight to degree and I'm sure there are a few colleges across the UK who offer very good A-level studies.
In my case I have applied both to the degree's I'd like to do, and a foundation course as a back up. Whilst the foundation course benefits a lot of people, I personally would not like to do it because I would feel that I would be somewhat repeating myself and possibly slowing things down as I am currently doing an Art&Design A-Level and a Photography one and have already done a lot of vocational work that is covered more within a degree than a foundation.
It's a personal choice really as at most institutes it is free (though I know it's not at LCF).
Hope that helped.