In THEORY, once an address range is delegated to you it is YOURS. CIDR
permits "holes", that is, more-specific routes.
Yes, this eventually will cause CIDR to fail due to entropy. What is new
about that? This was known and understood when CIDR was developed and
deployed.
Some providers try to force you to "give back" the address(es) when you
leave. MCSNet, and most others, do not. My view on this is that once you
receive an address consisting of at least a Class "C" block (ie: the last
octet is yours) then it is yours to keep -- period.
For sub-C allocations there is no good way to delegate those, and as such
at present we view sub-C allocations as belonging to us, and I suspect most
other providers who are as aggressive as we are in delegating small pieces
of address space also view things in this fashion.