Pros: It can make vehicle modes look beautiful. Just compare Binaltechs with Alternators; there's no contest. Metal feet can also help improve the base stability of a toy (e.g. MP22).

Cons: Paint may become chipped, although this is dependent on the quality of the paint. Good quality paint is much more resistant/durable. Die-cast metal is also apparently more expensive as a material, which is why (I think) Transformers phased out die-cast metal parts in 1986. This is why many 1986 Transformers have metal and plastic variants (e.g. metal toed Hot Rod, plastic toed Hot Rod et al.).

Mind you, the price differential becomes moot if they're going to charge approximately the same price for a plastic version of a toy as a metal one. I got metal-toed Hot Rod in 1986, but plastic-toed Kup... just as well I got Kup on clearance for $5, but still. The initial prices for Alternators in Australia was about the same as pre-ordering Binaltechs from HLJ. So naturally I just pre-ordered BTs from HLJ! Alternator prices did significantly go down later with sales, but when comparing full retail prices, I felt BTs gave me far better value for money. As for how much metal to be used, I think on most large vehicle panels is good (e.g. bonnet, roof, doors), but you don't want almost every part of the toy to be metal. That was something that I didn't like with Titanium Series.