Borrowing to invest is, by definition, multiplying your level of risk as well as your potential return. It might work out very well for you or it might go badly wrong. If you want to take that level of investment risk, away you go but it shouldn't be done with the assistance of tax relief from the government.
That said, they have only closed it off to one type of pension - the Occupational Pension Scheme. Borrowing is still possible in all other types of pensions - PRSAs, Personal Pensions, Buy Out Bonds.