I was asking on academia.se, if anyone knows scientific search engines offering a proximity operator like Google Web Search does, while Google Scholar Search does not. That's sad, because this operator would be most useful for literature research offering you a nearly semantic/context-sensitive search and I've seen requests for this feature in many blogs.
The answer on my question linked above shows that 2 search engines offer something similar, but those operators also only work on titles and abstracts of papers, if I understand correctly.
The wikipedia article doesn't explain what exactly limits the implementation of this kind of operator (exponentially rising indexing time, index size,... I'm no search algorithm expert), but when Google Web Search offers it (the amount of web-text is much bigger), what possibly hinders the scientific search engines from offering it for full article text (cost-benefit ratio? I doubt this, as 99,9% of Google Web Search user don't know the AROUND(X) operator and the majority doesn't use many operators at all)?
PS: If this question better fits SO, move it there, but I'm more looking for a general explanation, what parameters determine and limit the implementation of an proximity operator.