I'd say it has a few main facets: - ICMP is dangerous, and needs to be blocked. This is partly true, as there is historically information leakage with ICMP under certain codes. So blocking all of it is easier than trying to figure out what should be allowed and what should not.
- It's not perceived to be a problem. Most services just work, so TCP/IP with mss clamping will avoid the fragmentation issue for many services. So the services that have an issue, might be something like 2%, and the question that always lingers is if everything else works, and this one thing doesn't, it's not the filters problem.
- ICMP doesn't necessarily work with multiple layers of encapsulation anyways I encountered this on mobile wireless networks, where doing multiple rounds of encapsulation caused multiple packet size reductions. Basically, if we encapsulate in tunnels used by the mobile network, then encapsulate again in say IPSec, the IPSec node will send it's icmp packet too big to the tunnel endpoint, not the originator of the traffic.
While I suppose it could be argued that the tunnel endpoint should maintain it's own PMTU, and pro-actively send it's own icmp packet too big based on this, it can be difficult to guarantee these components work.
I've seen lots of telco equipment struggle with handling this properly, and it's not just filtering, it's bugs.
So unfortunately, we're in a situation where many networks are likely miss-configured, but lots of other services for various reasons just work. So unless you can drive the networks with problems to actually change, realistically you just lose a portion of your own customer base if you don't handle the situation while your competitors just work.