I did transfer my cash ISA to stocks and shares.
But it's not suitable for everyone.
For one thing cash ISAs save pretty much everyone tax. Share ISAs - not so much. This is why the government wants you to transfer - because cash ISAs cost more in lost tax.
Gordon Brown fucked over non-taxpayers, pensioners, etc. back in 1997, when he scrapped Advance Corporation Tax relief, meaning that you could no longer get dividend tax relief (on the tax paid on the underlying earnings).
So for Joe Average, shares typically don't have any tax payable at all - no income tax on dividends (unless you are a higher rate tax payer, and even then many shares are non income generating), and the capital gains tax allowance is, rather absurdly, higher than the personal allowance, so you really need to have a decent sized portfolio to have to worry about CGT.
Personally I use my share ISA for high yield investments, such as bonds. I bought Invesco Perpetual Monthly Income Plus in February, and they've since risen by 30% (increase in my capital value - basically due to improved sentiment about the risk of default by the bond issuers), and I'm still holding them, because they're still giving 8.16% yield, which does indeed piss all over anything you'd get from the bank.
It's not true BTW to say that you can only invest in FTSE250 companies, you can invest in pretty much any worldwide equity, property funds, whatever, some of which are very high risk. I invested in Russia in December 2007, they are down 39% since purchase, and this after rising 50% in the last six months!
It's best to avoid withdrawing from ISAs, because you lose the allowance forever. As a tax shelter, ISAs are potentially very valuable. Assuming a relatively low return of 4% per annum, and 40% tax, a single year's ISA allowance (£3600) is worth £57.60.
If you consider that 4% yield to gross up the £57.60, then withdrawing £3600 from your cash ISA (or equally, failing to use your allowance), costs you £57.60/.04 = £1440, as a higher rate tax payer, or as a basic rate payer £720. So best to treat all ISAs as long-term savings - cash ISA or not