[luncheonmeat]

No. It is dangerous for all the users  both the wheelchair bound person,  and the others further below.  Why you think our government spent money building the lifts in MRT stations?
Wheelchair users and their caregivers  should take the lift !
Others, even elderly folks,  waiting for the lift  should give priority to them (but not necessarily for their other family and friends accompanying them ), regardless of who arrived first.

Dun think that is the covention lah. Lifts are for those who cannot take the escalator [some elderlies feel giddy cannot use escalator]. It is a case of 1st come 1st serve [even able-bodied users oso no give priority to wheelchairs].