Alt coins (regardless of status/ valuation) add much validity & weight to btc's relative value.
wut
Bitcoin is not the Ford of currencies -- that would be the $. With a 1 bil. $ market cap, it's not even a Crossley. Maybe a backyard special, encroaching on an established market.
The alts' relationship to Bitcoin is similar to Bitcoin's relationship to the dollar. Think of Bitcoin as a tick, leaching the $ economy. The alts then become a parasite's parasite, a tick-on-a-tick, sucking Bitcoin's blood. From the alt perspective, this relationship is strictly parasitic & unidirectional: The alts depend on Bitcoin, not the other way around.
Otherwise, a nice analogy.