The "If you prick us, do we not bleed?" test of personhood
I'd prefer the risk of an individual rich guy (or gal, in the case of Meg Whitman) spending a fortune to influence politics, over the risk of handing the polity over to corporations. The whole of EBay could assemble much more cash over a longer period than could Meg, to engage in politics if it found it to its benefit.
It's no slippery slope to restrict first amendment (or other consitutional) rights to corporations. It's definitely a slippery slope if you do extend such rights to paper entities, such as corporations. A corporation is a made-up thing, not a person.