The state should fund an adequate defence. When I practised criminal law we had a duty to protect and preserve the legal aid fund. I imagine the same holds now. Trouble is, there are no votes in such things so it takes an enlightened government to uphold the underlying values.
In the case of the Pistorius trail, the defence seems to have found the "best" (i.e. the most suited to their case rather than necessarily being the best qualified or most pre-eminent) expert witnesses some of whom presented evidence on comparatively tangential matters. The defence has done what it is supposed to do, throw the kitchen sink at the case in an attempt to get an acquittal. If this approach was generally taken with public money then teh legal aid fund would soon be depleted.
The problem I have with legal aid is that there is a comparatively limited supply of money whereas there is an almost entirely elastic demand. On one hand it shouldn't be the case that only the well off have access to justice but there is a risk that public money will be used to prosecute or defend relatively frivolous cases (which either should never have been brought and/or could have been resolved through arbitration).
I'm really torn in cases of parents trying to work out custody of the children where both parties are funded through legal aid. On one hand, when relationships break down I understand that children can become pawns and parents want what's best for them. On the other, I'm annoyed that public money is being wasted away because ex-partners cannot come to an agreement regarding their children.