The only time we use a scalp electrode over external CTG monitoring is if bub's heart rate is of a concern (say having some decels) and we need very good accurate monitoring and can't afford to loose it with the external CTG. The internal CTG where its attached to bubs head is more accurate if placed on correctly where as at times the external CTG can loose sections of bubs heart rate or swap to mums heart rate so your left wondering if its a true indication of what bubs heart rate is doing.
We also often used the scalp monitoring in multiple births if its difficult to get both babies (often you end up monitoring the one twice and the other misses out on the monitoring)
Also at times if the woman has a larger body habitus which makes it difficult to monitor the baby (especially if its OP or in a funny position as well) we will do a scalp electrode. In some woman in labour with more body tissue finding the heart rate can be near impossible without getting them to lie in a particluar position and during labour this can be near impossible especially when your doing it every 15minutes (with a doppler) some woman who are larger then the average you have no trouble so I don't mean to say its all bigger woman just sometimes this is an added factor as to why they may use a scalp clip instead.
I know some hospitals do it more routine then where I work. I'd say it wouldn't even be 1:50 who have a scalp clip where I work. They almost always have the external CTG if continuous monitoring is needed.