The Parliament serves a useful feature in that there are peaceful ways to handle conflicts between the crown and nobility.
Also, it severs the useful purpose of keeping some advisors near the king besides his household. Now, sometimes these "advisors" end up having all the power (Henry III...), but when they don't it must be remembered ideas are cheap... one can discard the bad ones and implement the good ones. This doesn't guarantee he'll chose the right ideas, but it at least gives him something to think about besides echochambers of yes men.
Also remember that some kings willingly delegated power just so they didn't have to think as much anymore!
I must say if we expect a mixture of good, average, and bad kings, having a parliament exist is good to keep the dynasty in power. Unless we get rid of bad kings, this institution actually helps the ruling dynasty.
Having it be too strong isn't so great for keeping the ruling dynasty in power (The House of Windsor is really cool, but honestly they aren't a powerful dynasty. The only time I can imagine them doing anything contrary to a PM's word is to put a break on changes if Britain elects genocidal MPs into a majority some decade down the line)