One might defend spending caps by pointing out that campaigns that spend more on advertising gain an "unfair" advantage. We don't want something as vulgar as money to influence our elections. But there will always be non-rational factors influencing voters; if not advertising exposure, then plain old name recognition. That's surely no better (and maybe worse; at least advertising has some informational content).
Rather than seeking to limit campaign spending, it might make sense to offer extra public funds if needed. (One possibly tricky question is how to set the qualifying criteria, though I guess they already do this somehow -- I'm not familiar with the details.) But spending caps just don't make any sense, unless you think that official campaign advertising has zero or negative informational content at the margin. (Maybe they just spread lies.)
But rather than directly funding the candidates, couldn't we find a better use of public funds, i.e. some form of political spending that would increase the rationality of the election more than subsidizing ads for the underdog? (That's not exactly asking a lot.) For example: how about funding ads for FactCheck.org and similar non-partisan civic groups? (I guess such groups always face the risk of partisan takeover, and the allocation of such funding would be highly contested, but surely there must be some transparently fair way to arrange this?) Or, better yet, institute Ackerman and Fishkin's idea of a nationwide "Deliberation Day":
Registered voters will be called together in neighborhood meeting places, in small groups of ﬁfteen, and larger groups of ﬁve hundred, to discuss the central issues raised by the campaign. Each deliberator will be paid $150 for the day's work of citizenship. ...
If Deliberation Day succeeded, everything else would change: the candidates, the media, the activists, the interest groups, the spin doctors, the advertisers, the pollsters, the fund raisers, the lobbyists, and the political parties. All would have no choice but to adapt to a more attentive and informed public