Would you be able to flash the OS you want on the phone if you don't want the Symbian or windows phone 7 on your phone?
while it might be technically possible- take the HTC HD2, for example -it would ruin the experience somewhat.
one thing that microsoft did with WP7 was dictate minimum features and hardware requirements which are intented to ensure that the devices are responsive and compatible. there are minimum specs on processor speed, megapixels for camera, screen size, onboard memory and other hardware items.
as a development platform it makes some tasks much simpler as you only need to design for one screen size (for now) and interacting with hardware is the same regardless which vendor's phone you develop for.
while the likes of the android have various entry points and capabilities, there are definately differences in the useability between devices and issues with upgrading releases due to the dependacny on vendors to update specifically for their hardware versions. its early times now, but that may prove to be a big issue in the long term.
the iphone has the consistency and useability and it does wonders for the brand, but at the cost of having no choice of vendor (which tends to keep the price up) and it causes a bit of contention, developer-wise, by locking out too much.