I'm a fan of herps anywhere in a zoo, but I am also a fan of taxonomic exhibits and I think a good reptile house is an effective way of displaying that class.
While including herps in zoogeographic exhibits isn't a bad thing, in my experience this approach often results in lower species diversity and numbers. Many zoogeographic complexes will only have 1-2 popular large reptiles. When smaller or more obscure species do get included, they can be overshadowed and kept in nondescript side tanks that get passed over by visitors.
Comparatively, even small reptile houses often boast a sizable number of species, and a well-designed reptile house sacrifices little for this increased diversity... and in my experience, other visitors will stop at most or every tank in a dedicated reptile house but will walk right past herp exhibits when there's a tiger or elephant across from them.
Also to second what
@Tim May said, concentrating similar animals in one part of a zoo is useful for visitors like myself; if I have limited time at a zoo and want to see reptiles more than anything, being able to see 80 or 90% of them in one building saves me a lot of time. I'm also more likely to remember that zoo for its reptiles if they are in one standout exhibit, rather than scattered piecemeal throughout the campus.