Welsh Rambler,
I have also heard these differing points of view regarding sim rides, but the mere fact that you are considering it as an option probably means (and no offence meant) that you are either a little rusty with instrument flying or unfamiliar with type. To me, both cases are strong enough to warrant some practice beforehand.
I have the assessment day on 21st March and have already booked the 737-200 sim at Real Simulation in Yorkshire for the prior day. It's a fair wedge to lay out (approx. £600 for 2 hours plus briefings etc.) on top of travel, accomodation and the £250 for Mr O' Leary's beer fund
, but compared with what I've already spent on training, it's small fry. I would hate not to get through selection due to being unprepared for the sim ride. At the end of the day you have to give yourself the best possible chance to get through and preparation in terms of personal/technical interview, sim ride and overall personal presentation is never wasted.
The bit about assessors wanting to see an improvement over the duration etc. Well hopefully, if you've prepared you'll be good on your first attempt and even better on your second, or at least as good. Ideally, you should be able to learn or observe something from every 'flight' you conduct. They can't & don't expect miracles from a low hours/not current on 737 pilot, but are looking for a reasonable standard of flying and good CRM skills etc.
Best of luck whatever you decide to do.
WTSS
