Some startups start with large customer bases but operate at a relatively low margin per customer (because its extremely cheap to serve each customer.) For instance, for $250 a month in hosting[1], I can service tens of millions of customers, but buying either of these services for that many customers would cost a lot more than the hosting.
Looking at real world metrics for one of my apps I'm currently collecting enough data points (for free using Flurry) that to use mix panel would cost a sizable (%15-%20) of the apps monthly revenue.
I'm not saying they're not worth it-- I'm sure they are. I just don't see a way for a startup to onramp here. Our primary burn is keeping us housed and fed, operations are being kept as cheap as possible until we have real revenue. $150 is a lot. (though kiss metrics has a $30 and $80 plans which alone means we're more likely to use them.)
[1] in part because hetzner's XE 4S servers are such a screaming deal- that's about 5 32GB servers.