I went to a primary school that had no uniform and was desperate to have one so my Mum purchased 'school clothes' which were schoolish i.e. pinafores and blouses or checked summer dresses but in a range of colours, styles etc.
Fast-forward to 2010 and DD1 starts school at a school that has an optional uniform. She's desperate to wear uniform, I'm not that bothered and so we compromise; she wears uniformish - pinafore/skirt with white blouse but any tights and hot pink Kickers (velcro!). Once a week, on the same day, she wears her non-uniformish clothes but only takes cardigans etc. that are labelled with her name. Uniform makes things more straightforward in this house and DD1 knows that we don't mind if whiteboard pens, brown poster paint and PVA glue and the like end up staining those items as that's part of what happens at school. I'd be less impressed if she ruined a more 'special' item of clothing by wearing it to school after she'd chosen to go for uniform.
DD2 starts nursery in September and she wants to wear uniform too! She's chosen a couple of pinafores and some blue t-shirts from Sainsbury along with a cardigan or two. The advantages are time-saving in terms of decision-making, only having to label one pile of clothes (as opposed to labelling everything that might ever be taken to school) and children not having to worry that we'll be cross if something is wrecked during an enthusiastic art session.
FWIW, if my DDs weren't into wearing uniforms they'd be in hand-me-down skirts/leggings with t-shirts and cardigans. Everything would be labelled, PE kit would be plain t-shirt and plain shorts, spare pair of socks (in case tights were worn on a PE day), cheap trainers/pumps, sunhat/cap, lightweight raincoat for the summer, warm coat with a hood for the other 11 months of the year, wellies/snowboots and comfy, practical shoes that have a decent grip and won't fall apart if they are used on the climbing wall/football pitch.