I am glad you have spent some more time with this game.
For me, the Parodius games offer some of the best things that Konami ever gave us: the designers/developers were able to transcend the "rules" of the more serious games and provide an interesting variety of options/creativity. It was truly a "mash-up" before it became trendy and commonplace.
--> --> -->THE REPLAY VALUE OF PARODIUS DA! (and its brethren) is the best part, in my opinion. A key to this, however, is that both Gradius and Twin Bee were ESTABLISHED SERIES. A mash-up like this works best in this situation, IMHO.
Sure, I replay the Gradius (Salamander) and Twin Bee games... but the sheer fun of replaying Parodius with different ships is enough to hug the damn HuCARD.
BROADENING MY OUTLOOK: I happen to really enjoy the Gradius series more than Twin Bee, so Parodius has only helped me gain more appreciation for Twin Bee*. I suspect that many folks who are unmoved by Gradius/Twin Bee still enjoy Parodius, thereby making Parodius the MAINSTREAM CROSSOVER HIT for folks that aren't hardcore shoot-em-up fans.
That's what I reckon, anyway.
*I played the hell out of Stinger (NES), but lost interest in Twin Bee during 16-bit generation because the THUNDERFORCES and SOLDIERS were ripe for playing.
DEEP THOUGHTS with Jack Esteban: I don't know how folks feel about it, but I always wondered what it would be like if you were allowed to switch ships on-the-fly (and offensive preferences): I don't like the fact that a HUGE part of the strategy (and fun) is from committing to a particular ship/attack preference, experimenting with stages, and determining the pros/cons of your choices. Now that I think about it, perhaps this is the LAST THING I would want in a Parodius game...