There's a fairly practical reason to ask for a breakup fee in a smaller acquisition (or even VC round): the legal costs of these events are significant for a small company. Generating Series A paperwork, or acquisition paperwork, could easily run 20-30k. That may not be a lot to a company generating millions in revenue, but to a small company just graduating Y Combinator (as an example), it could be a substantial amount of the money on hand.