We see the argument from both sides, so offer both with our Cambelt Kits and individually.
When fitted with care the caps don't tend to fall out - you can always get unlucky, but then you need to balance that with the dirt ingress issue with using a standard seal.
For future reference, we have the original MG Rover part for well under RRP here:
https://www.dmgrs.co.uk/products/kv6...8ab59fb6&_ss=r