There's no doubt that his on field leadership is inspiring. Having such a passionate leader inspires the rest of the team on. A club such as Manchester United, with such a strong culture, needs players like Neville on the field.
Neville can still put a good ball into the box. Although this is not entirely the be-all-and-end-all for a Wing back, it is still an added bonus that he can be relied upon to threaten the defence with a dangerous ball into the penalty area.
Although he is not as quick as his main contenders, Brown, O'Shea and Rafael, this means that he is more inclined to not go forward all the time, allowing whoever the winger is much more freedom to attack. Defensive cover is something that can not be underestimated, both on the wings and in the centre, and if Neville is providing cover for the winger, rather than pushing forward himself, then the effectiveness of an opposition counter-attack will be minimised.
A player as inspired as Neville will always give 100% for the team, and then some, and he is the type of player who really gets under the opposition's skin. The Scousers are particularly known for hating him, and so the effect that a player like him can have is that the opposition becomes pre-occupied about him, that they lose track of the task at hand.
When we face a club who boast a hostile atmosphere, the experience of guys like Neville, Giggs, Scholes etc. can help calm the rest of the team down, and keep them from making irrational mistakes. The match against Besiktas this year was a perfect example of this.
Brown, O'Shea and Rafael are all worthy candidates for the Right Back role, and each of them bring different elements to the side, however for the reasons mentioned above, I feel that Neville is, at this point in time, our best option at Right Back.