Edit: Another place you see this kind of arthimetic is crypto, but those specific use cases (Diffie Hellman, RSA, a few others) don't tend to be vectorized. You have one op you're trying to work through with large integers, and there's the carry dependency on each partial op. The carry depdent crypto algorithms aren't typically vectorisable.
And as for crypto, couldn't you use the vector registers to do scalar ops?
And there's tons of reasons why using one lane of vector units as scalar ops is an issue. Power, lack of ubiquitous vector units, their pipeline is commonly decoupled from the integer unit making branches goofy, etc.
