Comment by branko_d

That's true for seeks into the clustered (primary) index because that index includes all fields, so you don't need to "jump" to the heap to get them.

However, seeking into a secondary index, and then reading a column not included in that index incurs an additional index seek (into the clustered index), which may be somewhat slower than what would happen in a heap-based table.

So there are pros and cons, as usual...