one exception...you can be cleared to intercept and track the localizer and descend on the glideslope , maintain 3000'(for example ) and be told to expect approach clearance upon reaching 10 dme, or leaving 4000'.
but I agree...intercept and track inbound on the localizer is not a clearance to descend (unless additional verbiage included in clearance) or an approach clearance.
now if you are told to intercept and track localizer, descend and maintain 2000'...you could follow the glideslope, but you can't go below the cleared altitude. however, all altitude changes are usually assumed to be made at best rate