Optional Specialties other than IQ-based

[Nedorus]

I have a question about Optional Specialties.

The Basic Set rules explicitly restrict optional specialties to IQ-based skills.

Originally Posted by Characters B169

You may only do this with an Average or harder IQ-based skill

I was wondering whether as a GM I should "handwave" this in my campaign and what impact it would / could have.

The particular case where this came up is the Per-based Urban Survival skill.

The character in question ‘grew up’ as a transient hitching rides (often as a blind passenger) between space stations and space ports. She has panhandling and related skills to ‘get by’ in such places, often having run-ins with "the law" etc.
For me a specialty of Urban Survival (Starport) would make perfect sense.

I'd like to ask the question a bit broader though. In which cases would you allow / house-rule the skill specialties rule? Where would you allow / disallow Optional Specialties and why?

Any thoughts?