I don't think we need to characterize these companies as pump-and-dump schemes (although some are)-- we can give them the benefit of the doubt and say they're adopting a particular strategy of getting as big as possible by whatever means possible in order to increase acquisition value. This is an insanely risky strategy which wholly betrays whatever lip service founders and their investors have paid to "de-risking" the company in its early days.
Consider the elements of the gamble:
1) An acquirer exists
2) The acquirer has a reason for wanting to acquire the company
3) The acquirer has the money to do so
4) The deal will actually close without being scuttled during negotiations by one of the parties (or the startup's VC's)
5) 1-4 will happen before the next tech crash
6) As a founder, you will still have enough equity at the time of (5) for all of it to be financially worthwhile.
If any one of 1-5 goes wrong, you'll be left without a chair when the music stops.
(6) may fuck you even if 1-5 happen.