We use the bid/ask spread when you place market orders - not the "last" trade price.
Let's look at an example. At the instant you submit an order, it will look like this:
Thu Jun 16 09:38:31 2016: B:3.5500, A:3.5600, L:3.6300
Notice how the bid/ask was different than the last.
Order is filled at the ask.
Note: Orders of seldom traded stocks, futures and options symbols are filled by Ask/Bid prices.