I must be blunt: Your thinking with respect to comment 1) is completely backwards. With respect to "esoteric services", Asterisk is superior to any closed PBX. Far from the "very basic call control" you mention, Asterisk allows a level of control that only the most expensive IVR scripting engines can dare to contemplate. While a proprietary PBX may claim to have dozens or even hundreds of so-called features, Asterisk is essentially limitless. I have yet to find a PBX that does not present a brick wall at some point; typically when the needs and/or creativity exceed the box that the PBXs designers have placed it in. With Asterisk, any limits that exist are merely milestones. If there's a need, it's a safe bet that someone is already pushing it to the next level.
With Asterisk, the more esoteric the need, the more its capabilities will shine.
On the cost side, the primary advantage of open-source software is that costs can be directed to areas where they deliver real value. Perhaps you will be able to find a closed PBX for under $1000, but it's capabilities cannot even begin to approach those of Asterisk, and it will typically require the customer to pay for features they neither want nor need.
As for PC reliability, there are many platforms that Asterisk (or more importantly, Linux) will run on. The hardware is a commodity, so platform decisions can be based on need. If I need a cheap PC platform, I can have it. If I need a NEBS-compliant or better platform (and have the budget that a similarly classed PBX would require), a five-nines reliable envirionment can be delivered.